Kinetics of activation of the P4 promoter of pBR322 by the Escherichia coli cyclic AMP receptor protein

The activation of transcription initiation from the P4 promoter of pBR322 by the Escherichia coli cyclic AMP receptor protein (CRP) has been investigated using a fluorescence abortive initiation assay. The effect of the cyclic-AMP/CRP complex on the linear P4 promoter was to increase the initial binding (KB) of RNA polymerase to the promoter by about a factor of 10, but the rate of isomerization of closed to open complex (kf) was unaffected. One molecule of CRP per promoter was required for activation, and the concentration of cyclic AMP producing half-maximal stimulation was about 7-8 microM. Supercoiling caused a 2-3-fold increase in the rate of isomerization of the CRP-activated promoter, but weakened the initial binding of polymerase by about one order of magnitude. The unactivated supercoiled promoter was too weak to allow reliable assessment of kinetic parameters against the high background rate originating from the rest of the plasmid.

Multiple cyclic AMP receptors are linked to adenylyl cyclase in Dictyostelium

cAMP receptor 1 and G-protein alpha-subunit 2 null cell lines (car1- and g alpha 2-) were examined to assess the roles that these two proteins play in cAMP stimulated adenylyl cyclase activation in Dictyostelium. In intact wild-type cells, cAMP stimulation elicited a rapid activation of adenylyl cyclase that peaked in 1-2 min and subsided within 5 min; in g alpha 2- cells, this activation did not occur; in car1- cells an activation occurred but it rose and subsided more slowly. cAMP also induced a persistent activation of adenylyl cyclase in growth stage cells that contain only low levels of cAMP receptor 1 (cAR1). In lysates of untreated wild-type, car1-, or g alpha 2- cells, guanosine 5'-O-'(3-thiotriphosphate) (GTP gamma S) produced a similar 20-fold increase in adenylyl cyclase activity. Brief treatment of intact cells with cAMP reduced this activity by 75% in control and g alpha 2- cells but by only 8% in the car1- cells. These observations suggest several conclusions regarding the cAMP signal transduction system. 1) cAR1 and another cAMP receptor are linked to activation of adenylyl cyclase in intact cells. Both excitation signals require G alpha 2. 2) cAR1 is required for normal adaptation of adenylyl cyclase. The adaptation reaction caused by cAR1 is not mediated via G alpha 2. 3) Neither cAR1 nor G alpha 2 is required for GTP gamma S-stimulation of adenylyl cyclase in cell lysates. The adenylyl cyclase is directly coupled to an as yet unidentified G-protein.

19F n.m.r. studies of conformational changes accompanying cyclic AMP binding to 3-fluorophenylalanine-containing cyclic AMP receptor protein from Escherichia coli

A fluorine-containing analogue of the cyclic AMP (cAMP) receptor protein (CRP) from Escherichia coli was prepared by biosynthetic incorporation of 3-fluorophenylalanine (3-F-Phe). 19F n.m.r. studies on this protein have provided direct evidence for cAMP-induced conformational changes not only within the cAMP-binding domain but also within the hinge region connecting the cAMP-binding domain to the DNA-binding headpiece. At 313 K, the 19F n.m.r. spectrum of [3-F-Phe]CRP showed five signals corresponding to the five phenylalanine residues as expected for a symmetrical dimer. Proteolysis of [3-F-Phe]CRP with subtilisin produced a fragment (the alpha-fragment) containing the cAMP-binding domain. The alpha-fragment contains all the phenylalanines except for Phe-136, a residue located in the hinge region. By comparing the 19F spectra of [3-F-Phe]CRP and its alpha-fragment, the signal for Phe-136 was assigned. The chemical shifts of the corresponding signals in the two spectra are similar, indicating that the alpha-fragment retains the structure it has in the intact protein. The largest cAMP-induced shift was observed for the signal from Phe-136 providing direct evidence for a conformational change in the hinge region. However, whereas binding of a single cAMP molecule to a CRP dimer is known to be sufficient to activate the DNA binding, the n.m.r. data indicate that the hinge region does not have the same conformation in both subunits when only one cAMP molecule is bound.

Amino acid substitutions in the Dictyostelium G alpha subunit G alpha 2 produce dominant negative phenotypes and inhibit the activation of adenylyl cyclase, guanylyl cyclase, and phospholipase C

Previous studies have demonstrated that the Dictyostelium G alpha subunit G alpha 2 is essential for the cAMP-activation of adenylyl cyclase and guanylyl cyclase and that g alpha 2 null mutants do not aggregate. In this manuscript, we extend the analysis of the function of G alpha 2 in regulating downstream effectors by examining the in vivo developmental and physiological phenotypes of both wild-type and g alpha 2 null cells carrying a series of mutant G alpha 2 subunits expressed from the cloned G alpha 2 promoter. Our results show that wild-type cells expressing G alpha 2 subunits carrying mutations G40V and Q208L in the highly conserved GAGESG (residues 38-43) and GGQRS (residues 206-210) domains, which are expected to reduce the intrinsic GTPase activity, are blocked in multicellular development. Analysis of down-stream effector pathways essential for mediating aggregation indicates that cAMP-mediated activation of guanylyl cyclase and phosphatidylinositol-phospholipase C (PI-PLC) is almost completely inhibited and that there is a substantial reduction of cAMP-mediated activation of adenylyl cyclase. Moreover, neither mutant G alpha 2 subunit can complement g alpha 2 null mutants. Expression of G alpha 2(G43V) and G alpha 2(G207V) have little or no effect on the effector pathways and can partially complement g alpha 2 null cells. Our results suggest a model in which the dominant negative phenotypes resulting from the expression of G alpha 2(G40V) and G alpha 2(Q208L) are due to a constitutive adaptation of the effectors through a G alpha 2-mediated pathway. Analysis of PI-PLC in g alpha 2 null mutants and in cell lines expressing mutant G alpha 2 proteins also strongly suggests that G alpha 2 is the G alpha subunit that directly activates PI-PLC during aggregation. Moreover, overexpression of wild-type G alpha 2 results in the ability to precociously activate guanylyl cyclase by cAMP in vegetative cells, suggesting that G alpha 2 may be rate limiting in the developmental regulation of guanylyl cyclase activation. In agreement with previous results, the activation of adenylyl cyclase, while requiring G alpha 2 function in vivo, does not appear to be directly carried out by the G alpha 2 subunit. Our data are consistent with adenylyl cyclase being directly activated by either another G alpha subunit or by beta gamma subunits released on activation of the G protein containing G alpha 2.

The cAMP-CRP/CytR nucleoprotein complex in Escherichia coli: two pairs of closely linked binding sites for the cAMP-CRP activator complex are involved in combinatorial regulation of the cdd promoter

Transcription initiation at CytR regulated promoters in Escherichia coli is controlled by a combinatorial regulatory system in which the cAMP receptor protein (CRP) functions as both an activator and a co-repressor. By combining genetic studies and footprinting analyses, we demonstrate that regulated expression of the CytR controlled cdd promoter requires three CRP-binding sites: a high affinity site (CRP-1) and two overlapping low affinity sites (CRP-2 and CRP-3) centred at positions -41, -91 and -93, respectively. In the absence of CytR, cAMP-CRP interacts at one set of sites (CRP-1 and CRP-2) and both of these binding sites are required for full promoter activation. In the presence of CytR, however, the two regulators bind cooperatively to cddP forming a nucleoprotein complex in which cAMP-CRP binds to CRP-1 and CRP-3 and CytR occupies the sequence between these sites. Thus, association of the two regulators involves a repositioning of the cAMP-CRP complex. Moreover, mutant cdd promoters in which CRP-2 and CRP-3 have been deleted are partially regulated by CytR, and cAMP-CRP and CytR still bind cooperatively to these promoters. These findings provide clues to an understanding of how cAMP-CRP and CytR interact at a structurally diverse set of promoters.

Inhibition of the self-renewal capacity of blast progenitors from acute myeloblastic leukemia patients by site-selective 8-chloroadenosine 3',5'-cyclic monophosphate

The physiologic balance between the two regulatory subunit isoforms, RI and RII, of cAMP-dependent protein kinase is disrupted in cancer cells; growth arrest and differentiation of malignant cells can be achieved when the normal ratio of these intracellular signal transducers of cAMP is restored by the use of site-selective cAMP analogs. In this study we evaluated the effects of the site-selective cAMP analog 8-chloroadenosine 3',5'-cyclic monophosphate (8-Cl-cAMP) on clonogenic growth of blast progenitors from 15 patients with acute myeloblastic leukemia and 3 patients affected by advanced myelodysplastic syndrome. Leukemic blast progenitors undergo terminal divisions, giving rise to colonies in methylcellulose. The self-renewal capacity of blast progenitors is conversely reflected in a secondary methylcellulose assay after exponential growth of clonogenic cells in suspension cultures. In all the samples tested, 8-Cl-cAMP, at micromolar concentrations (0.1-50 microM), suppressed in a dose-dependent manner both primary colony formation in methylcellulose and the recovery of clonogenic cells from suspension culture. Strikingly, in the samples from the entire group of patients, 8-Cl-cAMP was more effective in inhibiting the self-renewing clonogenic cells than the terminally dividing blast cells (P = 0.005). In addition, in four out of six cases studied, 8-Cl-cAMP was able to induce a morphologic and/or immunophenotypic maturation of leukemic blasts. An evident reduction of RI levels in fresh leukemic cells after exposure to 8-Cl-cAMP was also detected. Our results showing that 8-Cl-cAMP is a powerful inhibitor of clonogenic growth of leukemic blast progenitors by primarily suppressing their self-renewal capacity indicate that this site-selective cAMP analog represents a promising biological agent for acute myeloblastic leukemia therapy in humans.

cAMP-CRP activator complex and the CytR repressor protein bind co-operatively to the cytRP promoter in Escherichia coli and CytR antagonizes the cAMP-CRP-induced DNA bend

Initiation of transcription from the cytRP promoter in Escherichia coli is activated by the cAMP-CRP complex and negatively regulated by the CytR repressor protein. By combining gel retardation and footprinting assays, we show that cAMP-CRP binds to a single site centered at position -64 and induces a considerable bend in the DNA. CytR binds to a region immediately downstream from, and partially overlapping, the CRP site, and induces a modest bend into the DNA. In combination, cAMP-CRP and CytR bind co-operatively to cytRP forming a nucleoprotein complex in which the proteins directly interact with each other and bind to the same face of the DNA helix. CytR binding concomitantly antagonizes the cAMP-CRP-induced bend. This study indicates that the minimal DNA region required to obtain CytR regulation consists of a single binding site for each of cAMP-CRP and CytR. The case described here, in which a protein-induced DNA bend is modulated by a second protein, may illustrate a mechanism that applies to other regulatory systems.

Ligand-independent reduction of cAMP receptors in Dictyostelium discoideum cells over-expressing a mutated ras gene

Drug-resistance selection in Dictyostelium discoideum transformants resulted in up to eight-times-higher ras protein levels. Over-production of the wild-type ras protein did not lead to an aberrant phenotype. Increased levels of the mutated [G12T]ras protein, however, were correlated with severe deficiencies in aggregation and development. This aberrant phenotype is associated with reduced cAMP binding, due to a lower number of cell-surface receptors. We show that both RNA and cAMP-receptor-protein levels are reduced. These results indicate that ras in Dictyostelium discoideum seems to be involved in regulating cAMP-receptor-gene expression.

Anaerobic growth of Rhodopseudomonas palustris on 4-hydroxybenzoate is dependent on AadR, a member of the cyclic AMP receptor protein family of transcriptional regulators

The purple nonsulfur phototrophic bacterium Rhodopseudomonas palustris converts structurally diverse aromatic carboxylic acids, including lignin monomers, to benzoate and 4-hydroxybenzoate under anaerobic conditions. These compounds are then further degraded via aromatic ring-fission pathways. A gene termed aadR, for anaerobic aromatic degradation regulator, was identified by complementation of mutants unable to grow anaerobically on 4-hydroxybenzoate. The deduced amino acid sequence of the aadR product is similar to a family of transcriptional regulators which includes Escherichia coli Fnr and Crp, Pseudomonas aeruginosa Anr, and rhizobial FixK and FixK-like proteins. A mutant with a deletion in aadR failed to grow on 4-hydroxybenzoate under anaerobic conditions and grew very slowly on benzoate. It also did not express aromatic acid-coenzyme A ligase II, an enzyme that catalyzes the first step of 4-hydroxybenzoate degradation, and it was defective in 4-hydroxybenzoate-induced expression of benzoate-coenzyme A ligase. The aadR deletion mutant was unaffected in other aspects of anaerobic growth. It grew normally on nonaromatic carbon sources and also under nitrogen-fixing conditions. In addition, aerobic growth on 4-hydroxybenzoate was indistinguishable from that of the wild type. These results indicate that AadR functions as a transcriptional activator of anaerobic aromatic acid degradation.

Interactions between DNA-bound transcriptional regulators of the Escherichia coli gal operon

Regulation of the initiation of gene transcription from the gal operon of Escherichia coli is activated by the binding of CAP (catabolite activator protein) to a site centered at base pair -41.5 relative to the S1 start site of transcription. This operon is repressed by the specific binding of Gal repressor (GalR) to two operators, OE and OI, centered at -60.5 and +53.5, respectively. It has been proposed that this negative regulation results from the interaction of GalR dimers bound to OE and OI to form a protein-mediated "looped complex" [cf. Adhya, S. (1989) Annu. Rev. Genet. 23, 207-230]. In order to test whether DNA-bound CAP would facilitate or inhibit the binding of GalR, the simultaneous binding of these proteins was studied by quantitative DNase I footprint titration analysis. These studies demonstrate that GalR binding is noncooperative in the presence and in the absence of CAP and that GalR and CAP bind to the gal operon independently. No evidence was found that CAP stabilizes a putative Gal repressor-mediated protein-DNA looped complex. It has been shown that the gal operon can be negatively regulated by the binding of Lac repressor (LacI) to a gal operon in which OE and OI were both modified to be recognized by LacI [Haber, R., & Adhya, S. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 9683-9687]. In contrast to GalR, LacI binds to the chimeric gal operon with moderate cooperativity via the formation of a stable protein-DNA looped complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Photoaffinity labeling of regulatory subunits of protein kinase A in cardiac cell fractions of rats

The regulatory (R) subunits of adenosine 3',5',-cyclic monophosphate-dependent protein kinase were measured by photoaffinity labeling in heart tissue of rats flown on COSMOS 2044. Densitometric analyses showed a significant decrease of RII subunits in the particulate cell fraction extract (S2; P less than 0.05 in all cases) when extracts of tissue samples from vivarium controls were compared with those from flight animals. Photoaffinity labeling of the soluble fraction (S1) was unaffected by spaceflight or any of the simulation conditions. This was previously observed in heart muscle of rats flown on Spacelab 3 and COSMOS 1887 missions. A change in either the number or reactivity of the R subunits apparently results from some aspect of spaceflight. Proteins of the S2 fraction constitute a minor (less than 10%) component of the total, whereas the S1 fraction contained most of the cell proteins. No changes were seen when total incorporation of photoaffinity label was calculated on the basis of adrenal gland weights. A negative correlation resulted on comparison of controls with flight, synchronous control, and a tail-suspended simulation group of animals when incorporation of total counts due to azido labeling was based on body weights. Environmental factors that influence organismic responses may alter individual hormonal responses and may be reflected on the molecular level of organization. Conversely, changes in a relatively minor aspect of adenosine 3',5'-cyclic monophosphate-mediated reactions may be representative of a metabolic effect on an organismic level.

Escherichia coli dnaJ deletion mutation results in loss of stability of a positive regulator, CRP

The dnaJ deletion mutant K7052(lambda dnaK) has a temperature-sensitive defect in the synthesis of beta-galactosidase. We confirmed this operon-specific and temperature-sensitive defect in cell-free extracts prepared from the mutant cells and found that the missing factor was CRP. In the mutant, the cellular concentration of CRP was too low to allow the expression of the lac operon at a nonpermissive temperature. Introduction of a CRP over-producing plasmid into the dnaJ deletion mutant suppressed the defect of beta-galactosidase synthesis. The lower content of CRP in the mutant was found to result from extreme instability of the protein. These results strongly suggested that the heat shock protein dnaJ is involved in the stabilization (or degradation) of CRP.

Mechanism for iron control of the Vibrio fischeri luminescence system: involvement of cyclic AMP and cyclic AMP receptor protein and modulation of DNA level

Iron controls luminescence in Vibrio fischeri by an indirect but undefined mechanism. To gain insight into that mechanism, the involvement of cyclic AMP (cAMP) and cAMP receptor protein (CRP) and of modulation of DNA levels in iron control of luminescence were examined in V. fischeri and in Escherichia coli containing the cloned V. fischeri lux genes on plasmids. For V. fischeri and E. coli adenylate cyclase (cya) and CRP (crp) mutants containing intact lux genes (luxR luxICDABEG), presence of the iron chelator ethylenediamine-di(o-hydroxyphenyl acetic acid) (EDDHA) increased expression of the luminescence system like in the parent strains only in the cya mutants in the presence of added cAMP. In the E. coli strains containing a plasmid with a Mu dl(lacZ) fusion in luxR, levels of beta-galactosidase activity (expression from the luxR promoter) and luciferase activity (expression from the lux operon promoter) were both 2-3-fold higher in the presence of EDDHA in the parent strain, and for the mutants this response to EDDHA was observed only in the cya mutant in the presence of added cAMP. Therefore, cAMP and CRP are required for the iron restriction effect on luminescence, and their involvement in iron control apparently is distinct from the known differential control of transcription from the luxR and luxICDABEG promoters by cAMP-CRP. Furthermore, plasmid and chromosomal DNA levels were higher in E. coli and V. fischeri in the presence of EDDHA. The higher DNA levels correlated with an increase in expression of chromosomally encoded beta-galactosidase in E. coli and with a higher level of autoinducer in cultures of V. fischeri. These results implicate cAMP-CRP and modulation of DNA levels in the mechanism of iron control of the V. fischeri luminescence system.

Binding of the cyclic AMP receptor protein of Escherichia coli and DNA bending at the P4 promoter of pBR322

The binding of the Escherichia coli cyclic AMP receptor protein (CRP) to its specific site on the P4 promoter of pBR322 has been studied by gel electrophoresis. Binding to the P4 site was about 40-50-fold weaker than to the principal CRP site on the lactose promoter at both low (0.01 M) and high (0.1 M) ionic strengths. CRP-induced bending at the P4 site was investigated from the mobilities of CRP bound to circularly permuted P4 fragments. The estimated bending angle, based on comparison with Zinkel & Crothers [(1990) Biopolymers 29, 29-38] A-tract bending standards, was found to be approximately 96 degrees, similar to that found for binding to the lac site. These observations suggest that there is not a simple relationship between strength of CRP binding and the extent of induced bending for different CRP sites. The apparent centre of bending in P4 is displaced about 6-8 bp away from the conserved TGTGA sequence and the P4 transcription start site.

A ubiquitous factor is required for C/EBP-related proteins to form stable transcription complexes on an albumin promoter segment in vitro

The liver-enriched transcription factor CCAAT/enhancer binding protein (C/EBP) binds to numerous liver-specific promoters, yet the mechanism by which the protein stimulates transcription has not been described. The serum albumin promoter, which is liver specific, contains a strong C/EBP-binding site tightly juxtaposed to a binding site for the ubiquitous factor nuclear factor-Y (NF-Y). The binding of C/EBP impairs the binding of NF-Y; yet surprisingly, this arrangement leads to strong synergistic activation of a minimal promoter in liver nuclear extracts. Transcriptional synergism is manifested by NF-Y facilitating the ability of C/EBP to form preinitiation complexes that are stable through multiple rounds of transcription. Binding by itself, C/EBP stimulates transcription weakly without forming stable complexes, and moving the NF-Y binding site 10 bp away from the C/EBP site increases NF-Y binding in the presence of C/EBP but reduces the efficiency of stable complex formation and transcriptional synergism. These findings show that C/EBP requires precise positioning next to a ubiquitous factor for optimal formation of stable complexes and provides a model to understand the dramatic activation of the albumin gene in hepatic development.

cAMP-induced desensitization of surface cAMP receptors in Dictyostelium: different second messengers mediate receptor phosphorylation, loss of ligand binding, degradation of receptor, and reduction of receptor mRNA levels

Surface cAMP receptors on Dictyostelium cells are linked to several second messenger systems and mediate multiple physiological responses, including chemotaxis and differentiation. Activation of the receptor also triggers events which desensitize signal transduction. These events include the following: 1) loss of ligand binding without loss of receptor protein; 2) phosphorylation of the receptor protein, which may lead to impaired signal transduction; 3) redistribution and degradation of the receptor protein; and 4) decrease of cyclic AMP (cAMP) receptor mRNA levels. These mechanisms of desensitization were investigated with the use of mutant synag7, with no activation of adenylyl cyclase; fgdC, with no activation of phospholipase C; and fgdA, with defects in both pathways. cAMP-induced receptor phosphorylation and loss of ligand binding activity was normal in all mutants. In contrast, cAMP-induced degradation of the receptor was absent in all mutants. The cAMP-induced decrease of cAMP-receptor mRNA levels was normal in mutant synag7, but absent in mutant fgdC. Finally, the cAMP analogue (Rp)-cAMPS induced loss of ligand binding without inducing second messenger responses or phosphorylation, redistribution, and degradation of the receptor. We conclude that 1) loss of ligand binding can occur in the absence of receptor phosphorylation; 2) loss of ligand binding and receptor phosphorylation do not require the activation of second messenger systems; 3) cAMP-induced degradation of the receptor may require the phosphorylation of the receptor as well as the activation of at least the synag7 and fgdC gene products; and 4) cAMP-induced decrease of receptor mRNA levels requires the activation of the fgdC gene product and not the synag7 gene product. These results imply that desensitization is composed of multiple components that are regulated by different but partly overlapping sensory transduction pathways.

Promoter elements required for positive control of transcription of the Escherichia coli uhpT gene

The uhpABCT locus of Escherichia coli encodes the transport system which allows the cell to accumulate a variety of sugar phosphates in unaltered form. The expression of uhpT, the gene encoding the transport protein, is regulated by the uhpABC gene products. The UhpA protein is required for expression; its deduced amino acid sequence shows that it belongs to a subfamily of bacterial transcription regulators including NarL, DegU, and FixJ. Members of this subfamily have an amino-terminal phosphorylation domain characteristic of so-called two-component regulators, such as OmpR, CheY, PhoB, and NtrC, and a carboxyl-terminal domain conserved among many transcriptional activators, including LuxR and MalT. The major sequence elements in the uhpT promoter that are needed for uhpT expression were investigated. Northern (RNA) hybridization analysis showed that the uhpT transcript was only present in cells induced for UhpT transport activity. The start site of transcription was identified by primer extension. Comparison of the regions upstream of the uhpT transcription start site in E. coli and Salmonella typhimurium suggested the presence of four sequence elements that might be involved in promoter function: a typical -10 region, a short inverted repeat centered at -32, a long inverted repeat centered at -64, and a cyclic AMP receptor protein-binding sequence centered at -103. Deletion and linker substitution mutations in the promoter demonstrated that the presence of the cyclic AMP receptor protein-binding site resulted in about an eightfold increase in promoter activity and that the -64, -32, and -10 elements were essential for promoter function. In vivo titration of transcriptional activator UhpA by the intact or mutant promoters on multicopy plasmids identified the -64 element as the UhpA-binding site. The two halves of the -64 inverted repeat did not contribute equally to promoter function and did not have to be intact for UhpA titration. The sequence recognized by UhpA is predicted to be 5' -GGCAAAACNNNGAAA.

Unhydrolyzable analogues of adenosine 3':5'-monophosphate demonstrating growth inhibition and differentiation in human cancer cells

A set of adenosine 3':5'-monophosphate (cAMP) analogues that combine exocyclic sulfur substitutions in the equatorial (Rp) or the axial (Sp) position of the cyclophosphate ring with modifications in the adenine base of cAMP were tested for their effect on the growth of HL-60 human promyelocytic leukemia cells and LS-174T human colon carcinoma cells. Both diasteromeres of the phosphorothioate derivatives were growth inhibitory, exhibiting a concentration inhibiting 50% of cell proliferation of 3-100 microM. Among the analogues tested, Rp-8-Cl-cAMPS and Sp-8-Br-cAMPS were the two most potent. Rp-8-Cl-cAMPS was 5- to 10-fold less potent than 8-Cl-cAMP while Sp-8-Br-cAMPS was approximately 6-fold more potent than 8-Br-cAMP. The growth inhibition was not due to a block in a specific phase of the cell cycle or due to cytotoxicity. Rp-8-Cl-cAMPS enhanced its growth-inhibitory effect when added together with 8-Cl-cAMP and increased differentiation in combination with N6-benzyl-cAMP. The binding kinetics data showed that these Sp and Rp modifications brought about a greater decrease in affinity for Site B than for Site A of RI (the regulatory subunit of type I cAMP-dependent protein kinase) and a substantial decrease of affinity for Site A of RII (the regulatory subunit of type II protein kinase) but only a small decrease in affinity for Site B of RII, indicating the importance of the Site B binding of RII in the growth-inhibitory effect. These results show that the phosphorothioate analogues of cAMP are useful tools to investigate the mechanism of action of cAMP in growth control and differentiation and may have practical implication in the suppression of malignancy.

The cyclic nucleotide specificity of three cAMP receptors in Dictyostelium

cAMP receptors mediate signal transduction pathways during development in Dictyostelium. A cAMP receptor (cAR1) has been cloned and sequenced (Klein, P., Sun, T. J., Saxe, C. L., Kimmel, A. R., Johnson, R. L., and Devreotes, P. N. (1988) Science 241, 1467-1472) and recently several other cAR genes have been identified (Saxe, C. L., Johnson, R., Devreotes, P. N., and Kimmel, A. R. (1991a) Dev. Genet. 12, 6-13; Saxe, C. L., Johnson, R. L., Devreotes, P. N., and Kimmel, A. R. (1991b) Genes Dev. 5, 1-8). We have expressed three receptor subtypes, cAR1, cAR2, and cAR3, in growing cells and have investigated their affinity and pharmacological specificity in a series of [3H]cAMP binding studies. In phosphate buffer, there were two affinity states of about 30 and 300 nM for cAR1 and 20 and 500 nM for cAR3 but no detectable affinity for cAR2. In the presence of 3 M ammonium sulfate, there was one affinity state of 4 nM for cAR1 and 11 nM for cAR2 and two affinity states of approximately 4 and 200 nM for cAR3. The relative affinities of 14 cyclic nucleotide derivatives were tested for each cAR in ammonium sulfate. These studies suggest a model (Van Haastert, P. J. M., and Kien, E. (1983) J. Biol. Chem. 258, 9636-9642) in which cAMP binds to all three receptor subtypes by maintaining hydrogen bond interactions at the N6 and O3' positions. Interactions at the exocyclic oxygens of cAMP varied between the receptors; cAR2 and cAR3 lacked a stereoselective interaction at the axial oxygen which was present in cAR1. The cleft, which binds the adenine ring of cAMP, was hydrophobic in cAR1 and cAR3 but relatively polar in cAR2. The analog specificity of cAR1 and cAR3 in phosphate buffer was similar to that measured in ammonium sulfate though the derivatives' relative affinity to cAMP was reduced. We conclude that these cAMP receptor subtypes can be distinguished by distinct pharmacological properties which will allow selective activation of each cAR during development.

Global conformational changes in allosteric proteins. A study of Escherichia coli cAMP receptor protein and muscle pyruvate kinase

One of the basic features in allosteric regulation involves long range transduction of information. Based on crystallographic data on protein systems that are regulated by allosteric mechanisms, a global conformational change has always been observed. It is, therefore, important and useful to correlate the cooperativity of global structural change with the mode of binding of the regulatory ligand. Two systems were chosen for study, namely Escherichia coli cAMP receptor protein and muscle pyruvate kinase, which show negative and positive cooperativity in the binding of allosteric ligands, respectively. Quantitative titration of the global structural change, monitored by a high precision analytical gel chromatography technique, was conducted as a function of allosteric effector concentration. The results obtained for cAMP receptor protein show that the protein undergoes contraction upon binding of cAMP. The decreases in Stokes radius associated with complex formation are 0.1 +/- 0.1 and 0.7 +/- 0.1 A when one and two cAMP-binding sites are filled, respectively. The results for the pyruvate kinase system show a concerted structural change that quantitatively match the predicted behavior based on equilibrium constants derived from the analysis of steady state kinetic data by a two-state model. Hence, for these two systems, these results show that negative and positive cooperativity are correlated with sequential and concerted modes of structural change, respectively.

Positive and negative control of ompB transcription in Escherichia coli by cyclic AMP and the cyclic AMP receptor protein

The ompB operon encodes OmpR and EnvZ, two proteins that are necessary for the expression and osmoregulation of the OmpF and OmpC porins in Escherichia coli. We have used in vitro and in vivo experiments to show that cyclic AMP and the cyclic AMP receptor protein (CRP) directly regulate ompB. ompB expression in an ompB-lacZ chromosomal fusion strain was increased two- to fivefold when cells were grown in medium containing poor carbon sources or with added cyclic AMP. In vivo primer extension analysis indicated that this control is complex and involves both positive and negative effects by cyclic AMP-CRP on multiple ompB promoters. In vitro footprinting showed that cyclic AMP-CRP binds to a 34-bp site centered at -53 and at -75 in relation to the start sites of the major transcripts that are inhibited and activated, respectively, by this complex. Site-directed mutagenesis of the crp binding site provided evidence that this site is necessary for the in vivo regulation of ompB expression by cyclic AMP. Control of the ompB operon by cyclic AMP-CRP may account for the observed regulation of the formation of OmpF and OmpC by this complex (N. W. Scott and C. R. Harwood, FEMS Microbiol. Lett. 9:95-98, 1980).

Restored DNA-binding of the cAMP-CRP activator complex reestablishes negative regulation by the CytR repressor in the deoP2 promoter in Escherichia coli

We have investigated in vivo the coupling between CytR regulation of the deoP2 promoter in Escherichia coli and the DNA-binding specificity of the cAMP-CRP (cAMP receptor protein) complex in order to obtain a more detailed picture of the role played by cAMP-CRP in CytR regulation. By introducing CRP proteins that exhibit an altered DNA binding specificity into a strain containing a mutant deoP2 promoter in which cAMP-CRP activation was decreased and CytR regulation completely abolished, we show that CytR regulation of this promoter can be reestablished by restored the DNA binding of the cAMP-CRP complex. Hence, CytR regulation of deoP2 can be modulated by simply varying DNA binding of cAMP-CRP. These data confirm the crucial role played by the cAMP-CRP activator complex in CytR regulation of the deoP2 promoter.

Molecular insights into eukaryotic chemotaxis

Many cells display directed migration toward specific compounds. The best-studied eukaryotic models of chemotaxis are polymorphonuclear leukocytes, which respond to formylated peptides and Dictyostelium amoebas, which respond to extracellular cAMP. In both cell types, chemoattractants bind to surface receptors that contain seven transmembrane domains and interact with G proteins. Some cells, such as fibroblasts, undergo chemotaxis toward compounds whose receptors lack this motif and transmit their signals by other mechanisms. The cytosolic changes elicited by chemoattractants include increased levels of cAMP, cGMP, inositol phosphates, and calcium. These changes are correlated with actin polymerization and other cytoskeletal events that result in preferential extension of pseudopods toward the chemoattractant. Dictyostelium cell lines in which specific genes have been disrupted have demonstrated the necessity of a cAMP receptor (cAR1) and a G protein alpha-subunit (G alpha 2) for responsiveness to cAMP. Other proteins, such as myosin heavy chain and several actin binding proteins, are dispensible although their absence does affect the details of chemotaxis. The disruption of other relevant genes and the genetic reconstitution of chemotaxis in cells lacking crucial proteins should reveal many clues about this complicated and fascinating process.

Abberant chemotaxis and differentiation in Dictyostelium mutant fgdC with a defective regulation of receptor-stimulated phosphoinositidase C

Dictyostelium cells use extracellular cyclic AMP both as a chemoattractant and as a morphogen inducing cell-type-specific gene expression. Cyclic AMP binds to surface receptors, activates one or more G-proteins, and stimulates adenylate cyclase, guanylate cyclase and phosphoinositidase C. Mutant fgdC showed aberrant chemotaxis, and was devoid of cyclic AMP-induced gene expression and differentiation. Both the receptor- and G-protein-mediated stimulation of adenylate cyclase and guanylate cyclase were unaltered in mutant fgdC as compared to wild-type cells. In wild-type cells phosphoinositidase C was activated about twofold by the cyclic AMP receptor. In mutant fgdC cells, however, the enzyme was inhibited by about 60%. These results suggest that phosphoinositidase C is regulated by a receptor-operated activation/inhibition switch that is defective in mutant fgdC. We conclude that activation of phosphoinositidase C is essential for Dictyostelium development.

A cAMP-binding ectoprotein in the yeast Saccharomyces cerevisiae

Purified plasma membranes from the yeast Saccharomyces cerevisiae bind about 1.2 pmol of cAMP/mg of protein with high affinity (Kd = 6 nM). By using photoaffinity labeling with 8-N3-[32P]cAMP, we have identified in plasma membrane vesicles a cAMP-binding protein (Mr = 54,000) that is present also in bcy1 disruption mutants, lacking the cytoplasmic R subunit of protein kinase A (PKA). This argues that it is genetically unrelated to PKA. Neither high salt, nor alkaline carbonate, nor cAMP extract the protein from the membrane, suggesting that it is not peripherally bound. The observation that (glycosyl)phosphatidylinositol-specific phospholipases (or nitrous acid) release the amphiphilic protein from the membrane, thereby converting it to a hydrophilic form, indicates anchorage by a glycolipidic membrane anchor. Treatment with N-glycanase reduces the Mr to 44,000-46,000 indicative of a modification by N-linked carbohydrate side chain(s). In addition to the action of a phospholipase, the efficient release from the membrane requires the removal of the carbohydrate side chain(s) or the presence of high salt or methyl alpha-mannopyranoside, suggesting complex interactions with the membrane involving not only the glycolipidic anchor but also the glycan side chain(s). Topological studies show that the protein is exposed to the periplasmic space, raising intriguing questions for the function of this protein.

Environmentally regulated algD promoter is responsive to the cAMP receptor protein in Escherichia coli

The environmentally activated algD promoter of Pseudomonas aeruginosa has been shown to be influenced by DNA supercoiling. It is believed that protein-induced bending or looping is required for this activation. We studied the role of Escherichia coli cAMP-CRP on algD promoter activation in E. coli and show that a functional CRP is required for this activation. We also demonstrate that the algD promoter is sensitive to glucose repression both in E. coli and P. aeruginosa. Deletion of a putative consensus CRP binding sequence upstream of the algD promoter renders the promoter non-responsive to glucose repression. The involvement of cAMP-CRP complex in the activation of the algD promoter in E. coli has been demonstrated directly through binding of a 255 base pair DNA fragment containing the putative consensus CRP binding sequence. Other fragments, upstream or downstream but without any consensus CRP binding sequence, did not show any binding with CRP. A CRP-like analogue, similar to that in Xanthomonas campestris, but capable of activating genes without forming a complex with cAMP, is believed to allow glucose repression in P. aeruginosa.

The cyclic AMP (cAMP)-cAMP receptor protein complex functions both as an activator and as a corepressor at the tsx-p2 promoter of Escherichia coli K-12

The tsx-p2 promoter is one of at least seven Escherichia coli promoters that are activated by the cyclic AMP (cAMP)-cAMP receptor protein (CRP) complex and negatively regulated by the CytR repressor. DNase I footprinting assays were used to study the interactions of these regulatory proteins with the tsx-p2 promoter region and to characterize tsx-p2 regulatory mutants exhibiting an altered response to CytR. We show that the cAMP-CRP activator complex recognizes two sites in tsx-p2 that are separated by 33 bp: a high-affinity site (CRP-1) overlaps the -35 region, and a low-affinity site (CRP-2) is centered around position -74 bp. The CytR repressor protects a DNA segment that is located between the two CRP sites and partially overlaps the CRP-1 target. In combination, the cAMP-CRP and CytR proteins bind cooperatively to tsx-p2, and the nucleoprotein complex formed covers a region of 78 bp extending from the CRP-2 site close to the -10 region. The inducer for the CytR repressor, cytidine, does not prevent in vitro DNA binding of CytR, but releases the repressor from the nucleoprotein complex and leaves the cAMP-CRP activator bound to its two DNA targets. Thus, cytidine interferes with the cooperative DNA binding of cAMP-CRP and CytR to tsx-p2. We characterized four tsx-p2 mutants exhibiting a reduced response to CytR; three carried mutations in the CRP-2 site, and one carried a mutation in the region between CRP-1 and the -10 sequence. Formation of the cAMP-CRP-CytR DNA nucleoprotein complex in vitro was perturbed in each mutant. These data indicate that the CytR repressor relies on the presence of the cAMP-CRP activator complex to regulate tsx-p2 promoter activity and that the formation of an active repression complex requires the combined interactions of cAMP-CRP and CytR at tsx-p2.

Cyclic AMP-receptor protein activity in rat preputial cells

Cells from the rat preputial gland--a type of sebaceous gland--exhibited specific responsiveness of cyclic 3',5'-adenosine monophosphate (AMP) dependent protein kinase to stimulation by agents that elevate intracellular cyclic AMP. Electron microscopy shows that the rat preputial gland resembles the human sebaceous gland, not only in terms of containing a sebocyte-like population of cells in an acinar arrangement at different maturational stages, but also in the morphology of its organelles such as abundant and sometimes atypical mitochondria, many perinuclear lysosomes with crystalline inclusions, lipid droplets of various sizes, and peroxisomes. Other cell types, among them duct and inflammatory cells, were evident in the tissue sections, but constituted a minor component. Responses to stimulation of the adenylate cyclase-protein kinase pathways were determined using preputial cells that had been both freshly dispersed and grown in monolayer culture. Stimulation with isoproterenol (IPR) or forskolin (FS) resulted in both cases in an increase of cyclic AMP binding of the regulatory (R) subunits of cyclic AMP-dependent protein kinase, as determined by photoaffinity labeling of R subunits with an azido analog of cyclic AMP ([32P]-8-azido cyclic AMP). Cells from the epidermis under comparable conditions responded to a lesser degree and with a different distribution of R subunit isoforms. There are, therefore, differences in receptor activity as well as in the transduction pathways between the two types of epithelial cell populations. These results indicate that the preputial gland contains precursor cells that differentiate in culture to retain specific molecular mechanisms of action mediated via cyclic AMP.

Kinetics and nucleotide specificity of a surface cAMP binding site in Dictyostelium discoideum, which is not down-regulated by cAMP

Dictyostelium cells exhibit four types of kinetically distinct surface cAMP binding sites, the AH, AL, BS, and BSS sites, which are down-regulated during persistent stimulation with cAMP. Although most cAMP-induced responses are subject to desensitization during constant stimulation, some responses, notably the induction of post-aggregative gene expression, require persistent cAMP stimulation. The kinetics and specificity of residual cAMP-binding activity in cells treated for 4 h with micromolar cAMP were investigated. It was found that around 4000 rapidly dissociating binding sites per cell with an affinity of about 300 nM are retained after down-regulation. The nucleotide specificity of the remaining sites was very similar, but not completely identical to the AH, AL and B sites, suggesting that these sites belong to the same class of cell surface cAMP receptors and may be utilized to mediate responses requiring continuous cAMP stimulation.

Involvement of cyclic AMP cell surface receptors and G-proteins in signal transduction during slug migration of Dictyostelium discoideum

The presence of G-proteins, interacting with cAMP surface receptors, was investigated in vegetative cells, aggregation-competent cells, and migrating slugs of Dictyostelium discoideum. Our results indicate that G-proteins are present in all stages. In vegetative cells there is a limited number of cAMP receptors but no effect of GTP tau S on cAMP binding could be detected; in addition, no effect of cAMP on GTP tau S binding or GTPase activity was observed. In both aggregation-competent cells and slugs GTP tau S inhibits cAMP binding, while cAMP stimulates GTP tau S binding and high-affinity GTPase. Since the presence of G-proteins coupled to cAMP receptors could be demonstrated in slugs, the involvement of the effector enzymes adenylate cyclase and phospholipase C was investigated. The results show that adenylate cyclase activity is stimulated by GTP tau S in both stages and that in cells from migrating slugs the Ins(1,4,5)P3 production is increased upon stimulation with cAMP. The possible involvement of G-proteins in signal transduction during the slug stage of D. discoideum is discussed.

Supercoiling is essential for the formation and stability of the initiation complex at the divergent malEp and malKp promoters

malEp and malKp are divergent and partially overlapping promoters of the Escherichia coli maltose regulon, whose activity depends on the presence of two transcriptional activators. MalT and CRP (cAMP receptor protein). Their activation involves a common 210 base-pair regulatory region encompassing multiple binding sites for both activators. Using a supercoiled plasmid containing malEp and malKp as template, purified proteins and a single-round transcription assay, we developed an in vitro system in which both promoters behave as in vivo. In this system, malEp and malKp are active only in the presence of both MalT and CRP, and various mutations in the MalT or CRP binding sites affect the promoters in the same way as they do in vivo. We showed that supercoiling plays a crucial role not only for the formation of the initiation complex at malEp and malKp but also for its stability. In addition, dimethylsulphate protection experiments provide evidence that the nucleoprotein complexes formed by CRP and MalT bound to malEp and malKp on supercoiled and relaxed DNA are different. We speculate that one of the roles of supercoiling might be to assist the assembly of a preinitiation complex involving the regulatory region DNA and several molecules of MalT and CRP.

Gene targeting of the aggregation stage cAMP receptor cAR1 in Dictyostelium

cAR1, a G-protein-linked surface cAMP receptor, plays a central role in the development of Dictyostelium. To investigate its role, we sought to target the cAR1 gene by homogolous recombination. Transformation of these amoebas with appropriately designed vectors results in integration into the cAR1 locus with high frequency. cAR1 "null" mutants, resulting from double crossover events, fail to bind or sense cAMP and arrest in early development. The null mutants can be rescued by constitutive expression of a wild-type cAR1 cDNA. Carboxy-terminal deletion mutants, derived from single crossover events, express a truncated form of cAR1 that binds and senses cAMP. These cells proceed through the developmental program, albeit with a delay. Constitutive expression of a similar truncated form of cAR1 also rescues the null mutant. These observations prove that cAR1-mediated signal transduction controls the development of Dictyostelium and allow structural/functional studies of a G-protein-linked receptor in its natural context.

A novel function of the cAMP-CRP complex in Escherichia coli: cAMP-CRP functions as an adaptor for the CytR repressor in the deo operon

Unlike classical bacterial repressors, the CytR repressor of Escherichia coli cannot independently regulate gene expression. Here we show that CytR binding to the deoP2 promoter relies on interaction with the master gene regulatory protein, CRP, and, furthermore, that cAMP-CRP and CytR bind co-operatively to deoP2. Using mutant promoters we show that tandem, properly spaced DNA-bound cAMP-CRP complexes are required for this co-operative binding. These data suggest that CytR forms a bridge between tandem cAMP-CRP complexes, and that cAMP-CRP functions as an adaptor for CytR. The implications of this new version of negative control in E. coli on bacterial gene expression and on combinatorial gene regulation in higher organisms are discussed.

Activation of a pertussis-toxin-sensitive guanine-nucleotide-binding regulatory protein during desensitization of Dictyostelium discoideum cells to chemotactic signals

The chemoattractant cAMP induces the activation of adenylate cyclase in Dictyostelium discoideum. Upon prolonged incubation with cAMP, cells become desensitized via two distinct processes: a decrease in the number of available cAMP-binding sites (down regulation) and modification of the receptor (presumably via phosphorylation) correlated with adaptation. These processes occur simultaneously, but differ in the cAMP dose dependency and reversibility. In this study we investigated the mechanism of adaptation; cells were incubated with a cAMP analog to induce desensitization mediated by adaptation. The cells were then washed, lysed and the interaction between cAMP, receptor, guanine-nucleotide-binding regulatory proteins (G proteins) and GTP was investigated. (1) cAMP receptors that are phosphorylated in vivo remain phosphorylated for at least 45 min after lysis. (2) Desensitization did not alter basal cAMP binding to the receptor nor the inhibitory effect of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) on this binding. (3) The stimulatory effect of cAMP on GTP[S] binding was also unchanged, while basal GTP[S] binding and the kinetics of binding were only slightly different. (4) Basal high-affinity GTPase activity was not altered but cAMP stimulation was reduced from 43 +/- 7% in control lysates to 14 +/- 4% in lysates from desensitized cells. (5) cAMP stimulation of GTPase was decreased by pretreatment of cells with pertussis toxin from 43 +/- 7% to 17 +/- 8% but this was not further altered in lysates from desensitized pertussis-toxin-treated cells. These observations indicate that during desensitization the phosphorylated receptor can still interact with G proteins. Furthermore, desensitization did not affect cAMP stimulation of GTP[S] binding but strongly reduced cAMP stimulation of GTPase, suggesting that a G protein becomes activated. This G protein is pertussis toxin sensitive and may be the inhibitor G protein (Gi). This would imply that adenylate cyclase desensitizes because Gi becomes activated.

Design of cAMP-CRP-activated promoters in Escherichia coli

We have studied the deoP2 promoter of Escherichia coli to define features that are required for optimal activation by the complex of adenosine 3',5' monophosphate (cAMP) and the cAMP receptor protein (CRP). Systematic mutagenesis of deoP2 shows that the distance between the CRP site and the -10 hexamer is the crucial factor in determining whether the promoter is activated by cAMP-CRP. Based on these observations, we propose that cAMP-CRP-activated promoters can be created by correctly aligning a CRP target and a -10 hexamer. This idea has been successfully tested by converting both a CRP-independent promoter and a sequence resembling the consensus -10 hexamer to strongly cAMP-CRP-activated promoters.

Regulation of glpD and glpE gene expression by a cyclic AMP-cAMP receptor protein (cAMP-CRP) complex in Escherichia coli

The glpE gene of E. coli was found to be transcribed divergently with respect to glpD, which is adjacent to glpE head-to-head on the E. coli chromosome. We constructed glpD- and/or glpE-lacZ fusion plasmids, which provided glpD and lacZ as reporter genes. The expression of glpD and glpE, under the control of the cAMP-CRP complex, was examined by measuring the activities in E. coli cells of beta-galactosidase encoded by lacZ and glycerol-3-phosphate dehydrogenase encoded by glpD. In the double-reporter-gene system, the expression of glpD and glpE was found to be positively regulated by cAMP-CRP. We also confirmed that intracellular levels of the translation products and the transcripts from glpD and glpE were positively regulated by cAMP-CRP. The cAMP-mediated induction of gene expression of glpD and glpE was significantly affected by structural alterations of the single CRP-binding site between glpD and glpE. These results indicate that the single CRP-binding site is a cis-acting element involved in the positive regulation of the expression of both glpD and glpE at the transcriptional level.

cAMP binding proteins in the rat cerebral cortex after administration of selective 5-HT and NE reuptake blockers with antidepressant activity

This study was undertaken to evaluate the cyclic adenosine monophosphate (cAMP) binding proteins in the cerebral cortex of rat after short- and long-term administration with antidepressants. Prolonged treatment with different antidepressants that inhibit serotonin or norepinephrine uptake such as fluoxetine and the (+) enantiomer of oxaprotiline, respectively, was able to induce an increase in the photoactivated incorporation of 8-N3-[32P]cAMP into a protein band with apparent molecular weight of 52,000 in both soluble and crude microtubule fraction. On the contrary, chronic treatment with the (-) enantiomer of oxaprotiline, which does not affect monoamine uptake, failed to produce this effect. Moreover, no changes were observed after acute or in vitro addition of antidepressants, suggesting that modification in the cAMP binding may be related to adaptive changes elicited by prolonged antidepressants treatment. In conclusion, our studies indicate that the cAMP binding protein associated with the crude microtubule fraction could be an intracellular target for the action of antidepressant drugs.

Regulation of protein phosphorylation in Dictyostelium discoideum

We have examined the phosphorylation of the cyclic adenosine 3':5' monophosphate (cAMP) cell surface chemotactic receptor and a 36 kDa membrane-associated protein (p36) in Dictyostelium discoideum. The activity of CAR-kinase, the enzyme responsible for the phosphorylation of the cAMP receptor, was studied in plasma membrane preparations. It was found that, as in intact cells, the receptor was rapidly phosphorylated in membranes incubated with [gamma 32P] adenosine triphosphate (ATP) but only in the presence of cAMP. This phosphorylation was not observed in membranes prepared from cells which did not display significant cAMP binding activity. cAMP could induce receptor phosphorylation at low concentrations, while cyclic guanosine 3':5' monophosphate (cGMP) could elicit receptor phosphorylation only at high concentrations. Neither ConA, Ca2+, or guanine nucleotides had an effect on CAR-kinase. It was also observed that 2-deoxy cAMP but not dibutyryl cAMP induced receptor phosphorylation. The data suggest that the ligand occupied form of the cAMP receptor is required for CAR-kinase activity. Although the receptor is rapidly dephosphorylated in vivo, we were unable to observe its dephosphorylation in vitro. In contrast, p36 was rapidly dephosphorylated. Also, unlike the cAMP receptor, the phosphorylation of p36 was found to be regulated by the addition of guanine nucleotides. Guanosine diphosphate (GDP) enhanced the phosphorylation while guanosine triphosphate (GTP) decreased the radiolabeling of p36 indicating that GTP can compete with ATP for the nucleotide triphosphate binding site of p36 kinase. Thus was verified using radiolabeled GTP as the phosphate donor. Competition experiments with GTP gamma S, ATP, GTP, CTP, and uridine triphosphate (UTP) indicated that the phosphate donor site of p36 kinase is relatively non-specific.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple genes for cell surface cAMP receptors in Dictyostelium discoideum

We have cloned and characterized three genes (CAR1, CAR2, CAR3) encoding potential cell surface, cyclic adenosine 3':5' monophosphate (AMP) receptors from Dictyostelium discoideum. The three proteins are predicted to be substantially similar in amino acid sequence throughout most of their transmembrane (TM) and loop domains but are distinctly different in their carboxyl terminal segments. In addition, all three genes possess an intron which interrupts an equivalent codon of TM3. CAR1 is expressed early in development when the cAMP relay system is being established. As development proceeds multiple size forms of CAR1 RNA are detected which apparently result from differences in their 5'-untranslated regions. Late in development levels of CAR1 RNA decrease. In contrast, CAR2 encodes a single sized RNA which is expressed only during postaggregative development. CAR3 expression is approximately 10% of CAR1 during early development, is maximal during tight aggregate formation but declines thereafter. Only one size class of CAR3 mRNA is detected throughout development. Because RNA for each of the three genes is present in postaggregative cells, it was of interest to determine the cell type distribution of each RNA. Gene-specific probes were hybridized to RNAs isolated from cells of Percoll gradient-enriched prespore and prestalk fractions and relative levels of hybridization compared. CAR1 and CAR3 show approximately the same pattern of accumulation; a 3-4 fold enrichment in prestalk cells. CAR2, however, is highly enriched in prestalk cells, more than 10 fold relative to prespore cells.

Studies of the cyclic adenosine monophosphate chemoreceptor of Paramecium

A doublet of proteins (approximately 48,000 Mr) from the Paramecium cell body membrane fits several criteria for the external cAMP chemoreceptor. These criteria include: (i) selective elution from a cAMP affinity column, matching a specificity that could be predicted from the behavioral response and whole-cell binding; (ii) binding to wheat germ agglutinin indicating the presence of carbohydrate moieties indicating surface exposure; and (iii) selective inhibition of the intact cells' chemoresponse to cAMP by antibodies against the doublet. Additional evidence for the existence of a receptor, in general, comes from selective elimination of the cAMP chemoresponse by photoaffinity labeling of while cells with 8-N3-cAMP. The doublet proteins are not identical to the regulatory subunit of a cAMP-dependent protein kinase from Paramecium, the Dictyostelium cAMP chemoreceptor, or the 42-45 kDa range proteins related to the large surface glycoprotein in Paramecium. The doublet proteins are not readily separable and, as in Dictyostelium, may represent two different covalent modification states of the same protein. Amino acid analysis indicates that the proteins are similar, but does not distinguish between the possibilities of proteolysis and covalent modification. Once cloned, this doublet may prove to be only the fifth external, eukaryotic chemoreceptor to be identified.

Mutations that alter the ability of the Escherichia coli cyclic AMP receptor protein to activate transcription

The effects of a number of mutations in the E. coli cyclic AMP receptor protein (CRP) have been determined by monitoring the in vivo expression and in vitro open complex formation at two semi-synthetic promoters that are totally CRP-dependent. At one promoter the CRP-binding site is centered around 41.5 base pairs upstream from the transcription start whilst at the other promoter it is 61.5 base pairs upstream. The CRP mutation E171K reduces expression from both promoters whilst H159L renders CRP totally inactive: neither mutation stops CRP binding at either promoter. The mutations K52N and K52Q reverse the effect of H159L and 'reeducate' CRP to activate transcription. CRP carrying both H159L and K52N activates transcription from the promoter with the CRP site at -41.5 better than wild type CRP. In sharp contrast, this doubly changed CRP is totally inactive with respect to the activation of transcription from the promoter carrying the CRP site at -61.5. Our results suggest that CRP can use different contacts and/or conformations during transcription activation at promoters with different architectures.

Posttranslational control of cyclic AMP-dependent protein kinase by phorbol ester in normal but not in chemically transformed 3T3 cells

Protein kinase C (PKC) and cyclic AMP-dependent protein kinase (PKA) are important for normal cell proliferation. We show that both kinases are down-regulated by a phorbol ester tumor promoter in nontumorigenic murine BALB/c 3T3 fibroblasts (A31 cells), whereas only PKC responded to the phorbol ester in a chemically transformed derivative cell line (DMBA31 cells). In quiescent A31 cells, phorbol dibutyrate (PDBu) caused a 10-fold reduction in PKC activity and a 5-fold reduction in PKA activity. In contrast, PDBu depleted quiescent DMBA31 cells of PKC activity only and had no effect on the PKA activity. In both cell lines, PDBu did not affect the level of PKA regulatory subunits (determined by a cyclic [3H]-AMP binding assay), the levels of mRNA encoding the catalytic and the abundant regulatory subunit (determined by Northern blotting), or the level of the catalytic subunit protein (determined by Western blotting). An in situ gel activity assay confirmed that PDBu reduced the kinase activity of native PKA complexes from A31 cells but not from DMBA31 cells. These results indicate that phorbol esters down-regulate PKA activity by a posttranslational mechanism. They further suggest that the activity of PKC and PKA may be coordinately regulated in nontumorigenic cells and that transformation can disrupt this coordinate regulation.

Interconversion of the DNA-binding specificities of two related transcription regulators, CRP and FNR

In Escherichia coli, FNR and CRP are homologous transcriptional regulators which recognize similar nucleotide sequences via DNA-binding domains containing analogous helix-turn-helix motifs. The molecular basis for recognition and discrimination of their target sites has been investigated by directed amino acid substitutions in the corresponding DNA-recognition helices. In FNR, Glu-209 and Ser-212 are essential residues for the recognition of FNR sites. A V208R substitution confers CRP-site specificity without loss of FNR specificity, but this has adverse effects on anaerobic growth. In contrast, changes at two (V208R and E209D) or three (V208R, S212G and G216K) positions in FNR endow a single CRP-site binding specificity. In reciprocal experiments, two substitutions (R180V and G184S) were required to convert the binding specificity of CRP to that of FNR. Altering Asp-199 in FNR failed to produce a positive control phenotype, unlike substitutions at the comparable site in CRP. Implications for the mechanism of sequence discrimination by FNR and CRP are discussed.

Properties of CAR-kinase: the enzyme that phosphorylates the cAMP chemotactic receptor of D. discoideum

The cell surface cAMP chemotactic receptor of D. discoideum can be phosphorylated in partially purified plasma membrane preparations in a ligand-dependent manner. CAR-kinase, the enzyme responsible for receptor phosphorylation, was shown to be an integral membrane protein. It could utilize either ATP or GTP to phosphorylate the receptor, although ATP was much more efficient. The apparent affinity constant for ATP was approximately 20-25 microM. Maximum CAR-kinase activity was observed between pH 6.5 and pH7, and required the presence of Mg2+. Neither Mn2+ nor Ca2+ could substitute for that divalent cation. The enzyme was found to be sensitive to the ionic strength and temperature of the incubation reaction. Dephosphorylation of the receptor was not observed in the membrane preparations, indicating that the enhanced level of receptor phosphorylation that occurred upon ligand binding was not an indirect reflection of receptor dephosphorylation and subsequent incorporation of radiolabeled phosphate.

The CytR repressor antagonizes cyclic AMP-cyclic AMP receptor protein activation of the deoCp2 promoter of Escherichia coli K-12

We have investigated the regulation of the Escherichia coli deoCp2 promoter by the CytR repressor and the cyclic AMP (cAMP) receptor protein (CRP) complexed to cAMP. Promoter regions controlled by these two proteins characteristically contain tandem cAMP-CRP binding sites. Here we show that (i) CytR selectively regulated cAMP-CRP-dependent initiations, although transcription started from the same site in deoCp2 in the absence or presence of cAMP-CRP; (ii) deletion of the uppermost cAMP-CRP target (CRP-2) resulted in loss of CytR regulation, but had only a minor effect on positive control by the cAMP-CRP complex; (iii) introduction of point mutations in either CRP target resulted in loss of CytR regulation; and (iv) regulation by CytR of deletion mutants lacking CRP-2 could be specifically reestablished by increasing the intracellular concentration of CytR. These findings indicate that both CRP targets are required for efficient CytR repression of deoCp2. Models for the action of CytR are discussed in light of these findings.

Localization of functional domains of the cAMP chemotactic receptor of Dictyostelium discoideum

The topography and functional domains of the cAMP chemotactic receptor of Dictyostelium discoideum were investigated by protease sensitivity to chymotrypsin. Proteolytic digestion of intact cells produced a 23-kDa fragment of the receptor that retained the photoaffinity label used to identify the receptor. Additionally, this fragment contained the sites phosphorylated by CAR-kinase, the enzyme that phosphorylates the ligand-occupied form of the receptor. The fragment was also found to be phosphorylated in response to cAMP stimulation of cells. Proteolytic digestion of either intact cells or membrane preparations did not appreciably alter the binding properties of the receptor, indicating that the domains which determine the cAMP binding pocket are likely to be transmembrane regions of the protein. Additionally, the sensitivity of down-regulated receptors to chymotrypsin digestion suggests that the initial loss of cAMP binding activity upon incubation of cells with high concentrations of ligand does not require receptor internalization.

Characterization of the binding of cAMP and cGMP to the CRP*598 mutant of the E. coli cAMP receptor protein

Wild type cAMP receptor protein (CRP) activates in vitro lac transcription only in the presence of cAMP. In contrast the mutant CRP*598 (Arg-142 to His, Ala-144 to Thr) can activate lac transcription in the absence of cyclic nucleotide or at concentrations of cAMP below that required by CRP. To further characterize the properties of CRP*598, the binding of cAMP and cGMP to CRP and CRP*598 has been determined. The intrinsic binding constant (K) values obtained for cAMP binding are: CRP, 1.9 x 10(4) M-1; CRP*598, 3.8 x 10(5) M-1. The K values obtained for cGMP binding are: CRP, 2.9 x 10(4) M-1; CRP*598, 2.7 x 10(4) M-1. The results indicate that the affinity of CRP and CRP*598 for cGMP is relatively unchanged while the affinity of CRP*598 for cAMP is approximately twenty times greater than that shown by CRP. Binding of cAMP by CRP and cGMP by CRP or CRP*598 exhibits slight negative cooperativity. The major difference seen is that CRP*598 binds cAMP with strong positive cooperativity. The importance of the unsubstituted N6 position of the adenine moiety is also shown by the similar affinity of both forms of CRP for N6-butyryl cAMP. The cAMP binding properties evinced by CRP*598 suggest that its intrinsically altered conformation may be related to that assumed by CRP in a CRP-DNA or a cAMP-CRP-DNA complex.