[The role of pulmonary hypertension on bronchopulmonary dysplasia]

50 years ago, Northway described Broncopulmonary Dysplasia (BPD) in preterm infants exposed to mechanical ventilation. Since then, their survival has increased, nevertheless a "new BPD" has appeared and its incidence has not diminished. One of the characteristics of this pathology is the the abnormal vascular remodeling, which in its most severe expression is known as Pulmonary Hyper tension (PH); with an incidence of 17% in patients with BPD, which is proportional to the severity of the disease (33% in severe BPD), and as mortality factor (up to 48% 2-year mortality in PH-BPD). Thereby, it is important to know the diagnostic methods and therapeutic alternatives, topics discus sed in this review. Considering the high mortality in BPD associated PH, screening strategies in at risk population become important. The gold standard is cardiac catheterization; however, transtho-rathic echocardiography is a useful tool for the screening and diagnosis of PH in displasic patients, using cuantitive measures and cualitative changes in the evaluation. Seric type-B natriuretic peptide has shown to be useful for follow-up; regarding images, CT scan is used in severe cases. In terms of therapy; inhaled Nitric Oxide as a pulmonary vasodilator, phosphodiesterase inhibitors -sildenafil-, endotelin antagonists -bosentan-, and prostacyclin analogues -iloprost-, have been proposed. Their use, dosis and treatment lenght still lack support of high quality evidence, but diverse clinical expe riences have been described. Interdisciplinary care is also important, highlighting to optimize nu trition. Therefore, the challenge is to effectively prevent BPD and its complications. A PH screening protocol should be associated with risk stratification and treatment guidelines.

Mange epizootic in white-nosed coatis in western Mexico

From November of 1994 to June of 1996 an epizootic of mange, probably caused by the mite Notoedres cati, occurred in white-nosed coatis (Nasua narica) in the tropical dry forests of the Chamela-Cuixmala Biosphere Reserve in western Mexico. A monitoring scheme to determine the extent and severity of the epizootic within coatis was implemented. Trapping periods and transects were conducted for 2 yr. To control the spread of the disease, all captured infected coatis were either euthanized or treated with acaricides such as Butox and Ivomec-F, depending on the severity of their infection. Four other species of wild mammals and feral cats had skin conditions resembling mange. A more severe problem with the disease was predicted and later confirmed in the less isolated areas of the reserve, with a higher density of coatis. Our results indicate that epizootics may be more prone to occur in areas with greater fragmentation and less isolation from anthropogenic influence. Interestingly, although there was an apparently severe impact of the mange epizootic in the coati population, the long-term impact of the disease is unknown but appears to be negligible. So in order to understand the role of diseases in wildlife populations, long-term experimental studies are required.

Effect of deletion of the major brain G-protein alpha subunit (alpha(o)) on coordination of G-protein subunits and on adenylyl cyclase activity

Heterotrimeric G-proteins, composed of alpha and betagamma subunits, transmit signals from cell-surface receptors to cellular effectors and ion channels. Cellular responses to receptor agonists depend on not only the type and amount of G-protein subunits expressed but also the ratio of alpha and betagamma subunits. Thus far, little is known about how the amounts of alpha and betagamma subunits are coordinated. Targeted disruption of the alpha(o) gene leads to loss of both isoforms of alpha(o), the most abundant alpha subunit in the brain. We demonstrate that loss of alpha(o) protein in the brain is accompanied by a reduction of beta protein to 32+/-2% (n = 4) of wild type. Sucrose density gradient experiments show that all of the betagamma remaining in the brains of alpha(o)-/- mice sediments as a heterotrimer (s20,w = 4.4 S, n = 2), with no detectable free alpha or betagamma subunits. Thus, the level of the remaining betagamma subunits matches that of the remaining alpha subunits. Protein levels of alpha subunits other than alpha(o) are unchanged, suggesting that they are controlled independently. Coordination of betagamma to alpha occurs posttranscriptionally because the mRNA level of the predominant beta1 subtype in the brains of alpha(o)-/- mice was unchanged. Adenylyl cyclase can be positively or negatively regulated by betagamma. Because the level of other alpha subunits is unchanged and alpha(o) itself has little or no effect on adenylyl cyclase, we could examine how a large change in the level of betagamma affects this enzyme. Surprisingly, we could not detect any difference in the adenylyl cyclase activity between brain membranes from wild-type and alpha(o)-/- mice. We propose that alpha(o) and its associated betagamma are sequestered in a distinct pool of membranes that does not contribute to the regulation of adenylyl cyclase.

G alpha(o) is necessary for muscarinic regulation of Ca2+ channels in mouse heart

Heterotrimeric G proteins, composed of G alpha and G betagamma subunits, transmit signals from cell surface receptors to cellular effector enzymes and ion channels. The G alpha(o) protein is the most abundant G alpha subtype in the nervous system, but it is also found in the heart. Its function is not completely known, although it is required for regulation of N-type Ca2+ channels in GH3 cells and also interacts with GAP43, a major protein in growth cones, suggesting a role in neuronal pathfinding. To analyze the function of G alpha(o), we have generated mice lacking both isoforms of G alpha(o) by homologous recombination. Surprisingly, the nervous system is grossly intact, despite the fact that G alpha(o) makes up 0.2-0.5% of brain particulate protein and 10% of the growth cone membrane. The G alpha(o)-/- mice do suffer tremors and occasional seizures, but there is no obvious histologic abnormality in the nervous system. In contrast, G alpha(o)-/- mice have a clear and specific defect in ion channel regulation in the heart. Normal muscarinic regulation of L-type calcium channels in ventricular myocytes is absent in the mutant mice. The L-type calcium channel responds normally to isoproterenol, but there is no evident muscarinic inhibition. Muscarinic regulation of atrial K+ channels is normal, as is the electrocardiogram. The levels of other G alpha subunits (G alpha(s), G alpha(q), and G alpha(i)) are unchanged in the hearts of G alpha(o)-/- mice, but the amount of G betagamma is decreased. Whichever subunit, G alpha(o) or G betagamma, carries the signal forward, these studies show that muscarinic inhibition of L-type Ca2+ channels requires coupling of the muscarinic receptor to G alpha(o). Other cardiac G alpha subunits cannot substitute.

G(O), a guanine nucleotide binding protein, is expressed during neurite extension in the embryonic mouse

The developmental pattern of expression of the G protein alpha o subunit and GAP43 were compared by immunohistochemical staining of mouse embryos. Staining for alpha o and GAP43 was identical and detected throughout the developing nervous system, and the antigens first appeared in neurons at the beginning of neuronal differentiation. GAP43 and alpha o were not detected in regions containing only neuroblasts. These observations suggest that alpha o and GAP43 may not be required for the decision to pass from neuroblast to differentiated neuron, but may play a role in signal transduction during early neuronal development.

An amino-terminal domain of the growth-associated protein GAP-43 mediates its effects on filopodial formation and cell spreading

GAP-43 is a neuronal protein that is believed to be important to neuronal growth and nerve terminal plasticity. It is enriched on the inner surface of growth cone membranes, a localization that may depend upon palmitoylation of Cys3 and Cys4. It is a major substrate for protein kinase C, which phosphorylates Ser41. Isolated GAP-43 can bind to actin and to calmodulin, and can activate the heterotrimeric GTP-binding proteins, G(o) and Gi. A peptide consisting of the GAP-43 sequence 39–55 binds calmodulin, and an amino-terminal GAP-43 (1–10) peptide activates G(o), suggesting that these stretches may be functional domains of the intact protein. When expressed in non-neuronal cells, GAP-43 enhances filopodial extension and has effects upon cell spreading. We have examined the effects of various GAP-43 domains upon this assay, by expression of GAP-43, GAP-43 mutant proteins, and GAP-43-CAT fusion proteins in COS-7 cells. We find that the amino terminus (Met-Leu-Cys-Cys-Met-Arg-Arg-Thr-Lys-Gln) is an important contributor to these effects on cell shape. A GAP-43 protein mutant in Cys3 and Cys4 does not bind to the membrane, and is inactive. Mutants in Arg6 or Lys9 also are inactive, although they remain localized to particulate fractions; Arg7 mutants are active. A chimeric gene consisting of GAP-43 (1–10) fused to chloramphenicol acetyl transferase (CAT) also causes cell shape changes. As for GAP-43, the effects of this fusion protein are abolished by mutations of Cys3, Cys4, Arg6 or Lys9, but not by mutation of Arg7. Therefore, the cell surface activity of transfected GAP-43 depends upon its amino terminus, although other domains may regulate it in this regard. Since the amino-terminal domain includes the peptide stretch known to be capable of activating G(o) and Gi, we examined the effect of GAP-43 on a Gi-regulated second messenger system, the inhibition of cAMP production in A431 cells. A431 cells stably transfected with GAP-43 spread less well than do controls. In addition, they evidence decreased levels of forskolin-stimulated cAMP, consistent with chronic stimulation of Gi. Stimulation of adenylate cyclase by isoproterenol reverses the GAP-43-induced changes in cell shape. This suggests that G protein stimulation is involved in GAP-43 effects upon cell shape.

Palmitoylation alters protein activity: blockade of G(o) stimulation by GAP-43

The addition of palmitate to cysteine residues enhances the hydrophobicity of proteins, and consequently their membrane association. Here we have investigated whether this type of fatty acylation also regulates protein-protein interactions. GAP-43 is a neuronal protein that increases guanine nucleotide exchange by heterotrimeric G proteins. Two cysteine residues near the N-terminus of GAP-43 are subject to palmitoylation, and are necessary for membrane binding as well as for G(o) activation. N-terminal peptides, which include these cysteines, stimulate G(o). Monopalmitoylation reduces, and dipalmitoylation abolishes the activity of the peptides. The activity of GAP-43 protein purified from brain also is reversibly blocked by palmitoylation. This suggests that palmitoylation controls a cycle of GAP-43 between an acylated, membrane-bound reservoir of inactive GAP-43, and a depalmitoylated, active pool of protein.

An intracellular guanine nucleotide release protein for G0. GAP-43 stimulates isolated alpha subunits by a novel mechanism

G protein-coupled membrane receptors activate G proteins by enhancing guanine nucleotide exchange. G0 is a major component of the growing regions (growth cones) of neurons. GAP-43 is a neuronal protein associated with the cytosolic face of the growth cone plasma membrane and stimulates binding of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) to Go (Strittmatter, S. M., Valenzuela, D., Kennedy, T. E., Neer, E. J., and Fishman, M. C. (1990) Nature 344, 836-841). Here we have examined the mechanism by which GAP-43 affects G0. Like G protein-coupled receptors, GAP-43 enhances GDP release from G0, increases the initial rate of GTP gamma S binding, and increases the GTPase activity of Go, all without altering the intrinsic kappa cat for the GTPase. Unlike the case for receptors, however, the GAP-43 effect is not blocked by pertussis toxin, nor affected by the presence or absence of beta gamma or of phospholipids. There is specificity to the interaction, in that GAP-43 increases GTP gamma S binding to recombinant alpha o and alpha i1, but not to recombinant alpha s. Thus, GAP-43 is a guanine nucleotide release protein with a novel mechanism of action, potentially controlling membrane-associated G proteins from within the cell.

Growth cone transduction: Go and GAP-43

The neuronal growth cone plays a crucial role in forming the complex brain architecture achieved during development, and similar nerve terminal mechanisms may operate to modify synaptic structure during adulthood. The growth cone leads the elongating axon towards appropriate synaptic targets by altering motility in response to a variety of extracellular signals. Independently of extrinsic clues, neurons manifest intrinsic control of their growth and form (Banker and Cowan, 1979). Hence, there must be intracellular proteins which control nerve cell shape, so-called 'plasticity' or 'growth' genes. GAP-43 may be such a molecule (Skene and Willard, 1981; Benowitz and Lewis, 1983). For example, GAP-43 is localized to the growth cone membrane (Meiri et al. 1986; Skene et al. 1986) and can enhance filopodial formation even in non-neuronal cells (Zuber et al. 1989a). It includes a small region at the amino terminus for membrane association and perhaps growth cone targeting (Zuber et al. 1989b, Liu et al. 1991). We have found that Go, a member of the G protein family that links receptors and second messengers, is the major non-cytoskeletal protein in the growth cone membrane (Strittmatter et al. 1990). Double staining immunohistochemistry for GAP-43 and Go shows that the distributions of the two proteins are quite similar. Purified GAP-43 regulates the activity of purified Go (Strittmatter et al. 1990), a surprising observation since GAP-43 is an intracellular protein. We have compared the mechanism of GAP-43 activation of Go with that of G protein-linked receptors.2+ interactions between Go and GAP-43 suggest that Go plays a pivotal role in growth cone function, coordinating the effects of both extracellular signals and intracellular growth proteins.

G0 is a major growth cone protein subject to regulation by GAP-43

G0, a GTP-binding protein that transduces information from transmembrane receptors, has been found to be a major component of the neuronal growth cone membrane. GAP-43, an intracellular growth cone protein closely associated with neuronal growth, stimulates GTP-gamma-S binding to G0. It does so through an amino-terminal domain homologous to G-linked transmembrane receptors. Thus, G0 in the growth cone may be regulated by intracellular as well as extracellular signals.

Preparation and characterization of monoclonal antibodies specific for human transforming growth factor alpha

A series of monoclonal antibodies (mAbs) against transforming growth factor alpha (TGF alpha) have been produced. The generation of these reagents, as well as their biochemical and immunochemical characterization is described. TGF alpha peptides, mutant recombinant TGF alpha proteins and two-site immunoradiometric assays were used to identify the epitopes recognized by each antibody. This approach has allowed the specific localization of immunodominant domains on the molecule. Certain mAbs were found to be useful for selected procedures. mAb 134A-2B3 was used for immunoblotting both the precursor and mature forms of TGF alpha from conditioned media of tumor cells. One mAb 189-2130.1, which reacted with the carboxyl terminal seventeen amino acids, was able to block TGF alpha binding to the EGF receptor. mAb 213-4.4 was used for immunohistochemical detection of TGF alpha in fixed tumor cells. mAbs 137-178 and 134A-2B3 were used to develop a two-site immunoradiometric immunoassay which was sensitive to 1 ng ml-1 and detected TGF alpha from a variety of tumor cells. A series of mAbs such as these could prove useful in studying the biochemical properties as well as the immunochemical localization of TGF alpha in normal tissues and tumors.

P22 repressor mutants deficient in co-operative binding and DNA loop formation

We show here, both in vivo and in vitro, that P22 repressor binds co-operatively to operator sites separated by an integral number of turns of the DNA helix. We measure this co-operativity in vivo using an assay in which repression of a promoter requires co-operative binding of P22 repressors to two separated (non-adjacent) operator sites. We report the isolation of mutant repressors that have high affinity for single operator sites, but are defective in co-operative binding. Six different mutants, all bearing single amino acid changes in the carboxyl domain, have been isolated. We purified the two mutants most deficient in co-operative binding, and found that they bind non-co-operatively in vitro to adjacent as well as to non-adjacent pairs of operator sites.

A repressor heterodimer binds to a chimeric operator

Replacement of the solvent-exposed residues of the DNA recognition helix of the 434 repressor with the corresponding residues of the P22 repressor generates a hybrid protein, 434R[alpha 3(P22R)], which binds specifically to P22 operators. We show here that a new DNA-binding specificity is generated by combining 434 and 434R[alpha 3(P22R)] repressor monomers to form a heterodimer. The heterodimer specifically recognizes a chimeric P22/434 operator that lacks two-fold rotational symmetry.

Single amino acid changes that render human IFN-alpha 2 biologically active on mouse cells

Human IFN-alpha 1 and IFN-alpha 2 differ in 28 of 166 amino acids and show very different specific antiviral activities on human and murine cells. We have identified, by hybrid scanning and site-directed mutagenesis, three residues in IFN-alpha 2, in positions 121, 125 and 132 which, when replaced individually or jointly by their IFN-alpha 1 counterparts, modify its activity on mouse cells by up to 400-fold. We argue that these residues are involved in direct contacts with the mouse interferon receptor.

Identification of peptide sequences at the tRNA binding site of Escherichia coli methionyl-tRNA synthetase

Four different structural regions of Escherichia coli tRNAfMet have been covalently coupled to E. coli methionyl-tRNA synthetase (MetRS) by using a tRNA derivative carrying a lysine-reactive cross-linker. We have previously shown that this cross-linking occurs at the tRNA binding site of the enzyme and involves reaction of only a small number of the potentially available lysine residues in the protein [Schulman, L. H., Valenzuela, D., & Pelka, H. (1981) Biochemistry 20, 6018-6023; Valenzuela, D., Leon, O., & Schulman, L. H. (1984) Biochem. Biophys. Res. Commun. 119, 677-684]. In this work, four of the cross-linked peptides have been identified. The tRNA-protein cross-linked complex was digested with trypsin, and the peptides attached to the tRNA were separated from the bulk of the tryptic peptides by anion-exchange chromatography. The tRNA-bound peptides were released by cleavage of the disulfide bond of the cross-linker and separated by reverse-phase high-pressure liquid chromatography, yielding five major peaks. Amino acid analysis indicated that four of these peaks contained single peptides. Sequence analysis showed that the peptides were cross-linked to tRNAfMet through lysine residues 402, 439, 465, and 640 in the primary sequence of MetRS. Binding of the tRNA therefore involves interactions with the carboxyl-terminal half of MetRS, while X-ray crystallographic data have shown the ATP binding site to be located in the N-terminal domain of the protein [Zelwer, C., Risler, J. L., & Brunie, S. (1982) J. Mol. Biol. 155, 63-81].(ABSTRACT TRUNCATED AT 250 WORDS)

Is sequence conservation in interferons due to selection for functional proteins?

The human alpha-interferon (IFN-alpha) gene family consists of at least 14 potentially functional non-allelic members; the amino acid sequences they encode differ from each other by up to approximately 20% of their residues. Human IFN-beta, which is encoded by a single gene, is distantly related to the IFN-alpha family; it differs in 67% of its residues from IFN-alpha 2. There is considerable evidence that IFN-alpha and -beta compete for the same receptors on their target cells. Comparison of 14 non-allelic human IFN-alpha sequences and the IFN-beta sequence has revealed that 37 of 166 residues are completely conserved and that several of these are arranged in clusters, for example at positions 29-33, 47-50 and 136-150. It is commonly held that evolutionary conservation of amino acids indicates that the residues in question are essential for function. To test this hypothesis in the case of IFNs, we have introduced single site-directed point mutations into the strictly conserved codons 48 and 49 of the IFN-alpha 2 gene which form part of the longest uninterrupted cluster (position 47-50). We report here that the mutant proteins, containing Tyr, Ser and Cys instead of Phe48, or His instead of Gln49, have biological activities indistinguishable from those of wild-type IFN-alpha. In addition, when Glu62, a residue conserved in all known alpha and beta IFNs of man, mouse and cattle, was replaced by Lys, antiviral activity remained unchanged.

Modification of specific lysine residues in E. coli methionyl-tRNA synthetase by crosslinking to E. coli formylmethionine tRNA

A protein affinity labeling derivative of E. coli tRNAfMet has been prepared which carries an average of one reactive side chain per molecule, distributed over four structural regions. Each side chain contains a disulfide bond capable of reaction with cysteine residues and an N-hydroxysuccinimide ester group capable of coupling to lysine epsilon-amino groups in proteins. Reaction of the modified tRNA with E. coli methionyl-tRNA synthetase leads to crosslinking only by reaction with lysine residues in the protein. Examination of the tRNA present in the crosslinked complex reveals that the enzyme is coupled to side chains attached to the 5' terminal nucleotide, the dihydrouridine loop, the anticodon and the CCA sequence. Digestion of the crosslinked enzyme with trypsin followed by peptide mapping reveals that the major crosslinking reactions occur at four specific lysine residues, with minor reaction at two additional sites. Native methionyl-tRNA synthetase contains 90 lysine residues, 45 in unique sequences of the dimeric alpha 2 enzyme. Crosslinking of the protein to different regions in tRNAfMet thus occurs with the high degree of selectivity necessary for use in determining the peptide sequences which are near specific nucleotide sequences of tRNA bound to the protein.

Production by Bacillis subtilis of brown sporulation-associated pigments

The mode of production of the brown pigments of Bacillus subtilis 168 L-4, pigments frequently used as phenotypic markers for sporulation in this organism, has been studied. A defined liquid medium which promoted maximal pigment formation was developed. Five brown components, which could be resolved by thin-layer chromatography, were produced in the culture broth. Removal of cells from the medium at the end of logarithmic growth did not alter the type or amount of the pigments formed, indicating that the cells excreted pigment precursors into the medium during growth. Pigment formation from the precursors was found to occur by an oxygen-requiring, base-dependent, Mn2+-requiring, nonenzymatic pathway. Pigment production was also stimulated by the presence of tyrosine and histidine in the medium. The increases in extracellular pH often associated with spore formation in B. subtilis might be the cause of the concomitant appearance of brown pigments.

Reversible inactivation of Escherichia coli methionyl-tRNA synthetase by covalent attachment of formylmethionine tRNA to the tRNA binding site with a cleavable cross-linker

Protein affinity labeling groups have been attached to single-stranded cytidine residues in four structural regions of tRNAfMet. Modification of the tRNA with an average of one cross-linking group per molecule is achieved with retention of 75% of the original methionine acceptor activity. Incubation of the modified tRNA with methionyl-tRNA synthetase (MetRS) results in covalent coupling of the protein and nucleic acid by reaction of N-hydroxysuccinimide ester groups attached to the tRNA with lysine residues in the enzyme. In the presence of excess MetRS, approximately 30% of the input tRNA can be covalently bound to protein, indicating that lysine residues are appropriately oriented for reaction with cross-linking groups attached to certain sites in the tRNA but not to others. The cross-linking reaction results in loss of aminoacylation activity of MetRS equal to the amount of covalently bound tRNA. Enzyme activity is restored by release of bound tRNA following cleavage of the disulfide bond of the cross-linker with a sulfhydryl reagent. The data indicate that cross-linking occurs at the tRNA binding site of the enzyme. In the presence of excess modified tRNAfMet, a maximum of 1 mol of tRNA is cross-linked per mol of MetRS, in keeping with the known anticooperative tRNA binding properties of the native dimeric synthetase. In addition, the coupling reaction is effectively inhibited by unmodified tRNAfMet, but not by noncognate tRNAs.

Rabbit reticulocyte initiation factor 2 contains two polypeptide chains of molecular weights 48,000 and 38,000

Eukaryotic initiation factor 2 (eIF-20) purified from rabbit reticulocyte lysates consists of equimolar amounts of two polypeptide chains of Mr 48,000 and 38,000. Determination of the molecular weight of the native factor gave a value which is consistent with a Mr of 86,000 indicating that the factor is composed of one Mr 48,000 and one Mr 38,000 polypeptide. The purified factor exhibited all the binding activities characteristic of eIF-2. The factor formed ternary complexes with Met-tRNAfMet and GTP; it bound GDP to form a binary complex; and it also possessed the property of binding a wide variety of RNA species, including reoviral mRNA, phage T3 mRNA, rRNAs, and tRNA. Furthermore, the ternary complex formed by purified eIF-2 interacted with the 40S ribosomal subunit in the presence of AUG codon to form a 40S initiation complex. These results indicate that all binding activities attributed to eIF-2 are contained in the 48,000- and 38,000-dalton polypeptides.

Hormonal control of gene expression: differential activation of rat bone marrow RNA polymerases by erythropoietin and testosterone

Hormones play a role in the regulation of gene expression by inducing changes in enzyme patterns in target cells mediated by the synthesis of specific RNA molecules. Erythropoiesis has been used as a system for studying the molecular mechanism of regulation of gene action by means of two hormones: erythropoietin and testosterone. Experiments designed to correlate the biochemical action of both hormones on rat marrow cells are herein reported. Both factors seems to act at different biochemical and citological levels. Erythropoietin triggers the erythropoietic process acting on the erythropoietin sensitive cells (ESC), in which the hormone induces the synthesis of a high molecular weight RNA, which is the precursor of a functional 9 S messenger RNA. Testosterone seems to act on polychromatophilic erythroblasts, in which the synthesis of ribosomal RNA or its precursor is stimulated. The steroid enhances the nuclear ribonuclease activity, which could represent a control mechanism for the processing (maturation) of high molecular weight RNAs. The incorporation of 3H-GTP and 3H-UTP into RNA by isolated rat bone marrow nuclei is stimulated by erythropoietin and testosterone. Using alpha-amanitine and different ionic strength conditions it was found that erythropoietin enhances preferentially RNA polymerase II activity while testosterone increases RNA polymerase I activity. It is postulated that erythropoietin and testosterone act synergically to create the biochemical machinery for hemoglobin synthesis, the macromolecule that characterizes the erythropoietic process.

Termination of transcription by Escherichia coli RNA polymerase: influence of secondary structure of RNA transcripts on rho-independent and rho-dependent termination

The effect of RNA secondary structure on rho-independent and rho-dependent termination of transcription of T3 DNA by Escherichia coli RNA polymerase has been studied by incorporating, into nascent transcripts, base analogs that lead to altered base-pairing properties. A guanine --> hypoxanthine substitution, with attendant weakening of secondary structure, abolished the rho-independent termination at 20% of the genome; in contrast, replacement of cytosine with 5-bromocytosine, which forms stronger pairs with guanine, enhanced termination at this site. rho-Independent termination was not altered by replacing uracil with 5-bromouracil. There are two major rho-dependent termination sites on the T3 DNA-at 8 and 15%. The termination activity of rho in this system also depended on RNA secondary structure. The incorporation of 5-bromouracil instead of uracil into RNA did not alter the site specificity of rho action but rho was rendered inactive when cytosine was replaced by 5-bromocytosine. In contrast, replacement of GTP with ITP in the reaction increased rho-dependent inhibition of RNA synthesis, caused production of heterogeneous-sized transcripts, and stimulated rho-mediated ATP hydrolysis. The rho-associated ATPase activity, in the presence of isolated T3 RNA, was also stimulated by inosine substitution. Furthermore, the temperature-sensitive rho isolated from rho 15 mutant of E. coli, which does not terminate transcription in the presence of the common rNTPs, was active when GTP was replaced with ITP. These results suggest that strongly paired G.C-rich regions in RNA stem-loop structures or RNA.DNA hybrids are essential for rho-independent termination, whereas rho-dependent termination requires weakly paired cytosine residues for its action.