The carboxy-terminal calcium binding sites of calmodulin control calmodulin's switch from an activator to an inhibitor of RYR1

Calcium and calmodulin both regulate the skeletal muscle calcium release channel, also known as the ryanodine receptor, RYR1. Ca(2+)-free calmodulin (apocalmodulin) activates and Ca(2+)-calmodulin inhibits the ryanodine receptor. The conversion of calmodulin from an activator to an inhibitor is due to Ca(2+) binding to calmodulin. We have previously shown that the binding sites for apocalmodulin and Ca(2+)-calmodulin on RYR1 are overlapping with the Ca(2+)-calmodulin site located slightly N-terminal to the apocalmodulin binding site. We now show that mutations of the calcium binding sites in either the N-terminal or the C-terminal lobes of calmodulin decrease the affinity of calmodulin for the ryanodine receptor, suggesting that both lobes interact with RYR1. Mutation of the two C-terminal Ca(2+) binding sites of calmodulin destroys calmodulin's ability to inhibit ryanodine receptor activity at high calcium concentrations. The mutated calmodulin, however, can still bind to RYR1 at both nanomolar and micromolar Ca(2+) concentrations. Mutating the two N-terminal calcium binding sites of calmodulin does not significantly alter calmodulin's ability to inhibit ryanodine receptor activity. These data suggest that calcium binding to the two C-terminal calcium binding sites within calmodulin is responsible for the switching of calmodulin from an activator to an inhibitor of the ryanodine receptor.

Ca2+-independent activity of nitric oxide synthase

Ca2+-independent forms of nitric-oxide synthase have significant activity when the endogenous calmodulin subunit is Ca2+ free. Further activation is seen when Ca2+ is added. We have examined the activation of a Ca2+-independent nitric-oxide synthase variant and its two point mutants that are more dependent on Ca2+ for activation using mutant calmodulins containing non-functional Ca2+-binding sites. These studies provide evidence that the Ca2+-independent activity of these enzymes can be exerted through specific adapted interactions between the enzyme and the Ca2+-binding site 2 of calmodulin. Further, the results suggest that EGTA-sensitive metals other than Ca2+ complexed to calmodulin may be involved in maximal activation of these nitric-oxide synthase variants.

Mutations at lysine 525 of inducible nitric-oxide synthase affect its Ca2+-independent activity

Calmodulin binding to inducible nitric-oxide synthase may play an important role in its Ca(2+)-independent activity. Studies of inducible nitric-oxide synthase chimeras containing the calmodulin binding sequence of neuronal or endothelial nitric-oxide synthases show that the calmodulin binding sequence of inducible nitric-oxide synthase is necessary but not sufficient for the Ca(2+)-independent activity. The mutations at lysine 525 located at the C terminus of the calmodulin binding sequence of inducible nitric-oxide synthase were examined for the effects on the Ca(2+)-independent activity with chimeras containing the oxygenase or reductase domains of inducible or neuronal nitric-oxide synthases. Results show that the Ca(2+)-independent binding of calmodulin is not solely responsible for maximal Ca(2+)-independent activity of inducible nitric-oxide synthase. Lysine 525 of inducible nitric-oxide synthase may also play an important role in coordinating the maximal Ca(2+)-independent activity.

An extended conformation of calmodulin induces interactions between the structural domains of adenylyl cyclase from Bacillus anthracis to promote catalysis

The edema factor exotoxin produced by Bacillus anthracis is an adenylyl cyclase that is activated by calmodulin (CaM) at resting state calcium concentrations in infected cells. A C-terminal 60-kDa fragment corresponding to the catalytic domain of edema factor (EF3) was cloned, overexpressed in Escherichia coli, and purified. The N-terminal 43-kDa domain (EF3-N) of EF3, the sole domain of edema factor homologous to adenylyl cyclases from Bordetella pertussis and Pseudomonas aeruginosa, is highly resistant to protease digestion. The C-terminal 160-amino acid domain (EF3-C) of EF3 is sensitive to proteolysis in the absence of CaM. The addition of CaM protects EF3-C from being digested by proteases. EF3-N and EF3-C were expressed separately, and both fragments were required to reconstitute full CaM-sensitive enzyme activity. Fluorescence resonance energy transfer experiments using a double-labeled CaM molecule were performed and indicated that CaM adopts an extended conformation upon binding to EF3. This contrasts sharply with the compact conformation adopted by CaM upon binding myosin light chain kinase and CaM-dependent protein kinase type II. Mutations in each of the four calcium binding sites of CaM were examined for their effect on EF3 activation. Sites 3 and 4 were found critical for the activation, and neither the N- nor the C-terminal domain of CaM alone was capable of activating EF3. A genetic screen probing loss-of-function mutations of EF3 and site-directed mutations based on the homology of the edema factor family revealed a conserved pair of aspartate residues and an arginine that are important for catalysis. Similar residues are essential for di-metal-mediated catalysis in mammalian adenylyl cyclases and a family of DNA polymerases and nucleotidyltransferases. This suggests that edema factor may utilize a similar catalytic mechanism.

Alternative splicing of Drosophila calcium/calmodulin-dependent protein kinase II regulates substrate specificity and activation

Drosophila calcium/calmodulin-dependent protein kinase II is alternatively spliced to generate multiple isoforms that vary only in a region between the calmodulin-binding domain and the association domain. This variation has been shown to modulate activation of the enzyme by calmodulin. In this study we examine the ability of seven of the Drosophila isoforms to phosphorylate purified protein substrates and to be inhibited by a substrate analog, and the response of six of the isoforms to a mutant form of calmodulin (V91G) that was isolated in a genetic screen. Significant variation in Kms for Eag, a potassium channel, and Adf-1, a transcription factor, were found. In the case of the a peptide inhibitor, AC3I, there were significant variations in Ki between isoforms. Kact for V91G calmodulin was increased for all of the isoforms. In addition, one isoform, RI, exhibited a lower Vmax when assayed with this mutant CaM. These results indicate that the variable domain of calcium/calmodulin-dependent protein kinase II is capable of altering the substrate specificity of the catalytic domain and the activation response to calmodulin.

Regulation of RYR1 activity by Ca(2+) and calmodulin

The skeletal muscle calcium release channel (RYR1) is a Ca(2+)-binding protein that is regulated by another Ca(2+)-binding protein, calmodulin. The functional consequences of calmodulin's interaction with RYR1 are dependent on Ca(2+) concentration. At nanomolar Ca(2+) concentrations, calmodulin is an activator, but at micromolar Ca(2+) concentrations, calmodulin is an inhibitor of RYR1. This raises the question of whether the Ca(2+)-dependent effects of calmodulin on RYR1 function are due to Ca(2+) binding to calmodulin, RYR1, or both. To distinguish the effects of Ca(2+) binding to calmodulin from those of Ca(2+) binding to RYR1, a mutant calmodulin that cannot bind Ca(2+) was used to evaluate the effects of Ca(2+)-free calmodulin on Ca(2+)-bound RYR1. We demonstrate that Ca(2+)-free calmodulin enhances the affinity of RYR1 for Ca(2+) while Ca(2+) binding to calmodulin converts calmodulin from an activator to an inhibitor. Furthermore, Ca(2+) binding to RYR1 enhances its affinity for both Ca(2+)-free and Ca(2+)-bound calmodulin.

Androcam, a Drosophila calmodulin-related protein, is expressed specifically in the testis and decorates loop kl-3 of the Y chromosome

The Drosophila genome encodes a protein that is 68% identical to Drosophila calmodulin (Cam). We show here that this Cam-related gene is specifically expressed in the germ-line of the testis, leading to the name Androcam (Acam). Early in spermatogenesis Acam accumulates on one of the chromatin loops of the Y chromosome, kl-3. This association with kl-3 may indicate an RNA processing-related role for Acam and/or could reflect an unusual storage/assembly function hypothesized for the Y loops. After meiosis Acam is detectable in developing sperm tail cytoplasm, where at least some of the protein is not tightly associated with tubulin. Late in spermiogenesis, some Acam staining overlaps the periphery of the investment cones, actin-containing structures hypothesized to support the motor function for cytoplasmic stripping of the tail. Acam cannot be detected in mature sperm by immunolocalization, but immunoblotting established that Acam is present in sperm stored in mated females, suggesting epitope masking during final maturation. Proteins more related to Acam than Cam are present in the testes of other Drosophila species and a mammalian species, the mouse.

bicaudal encodes the Drosophila beta NAC homolog, a component of the ribosomal translational machinery*

bicaudal was the first Drosophila mutation identified as producing mirror-image pattern duplications along the anteroposterior axis of the embryo. However the mutation has been little studied due to its low penetrance and suppressibility. We undertook cloning of the bicaudal locus together with studies of the mutation's effects on key elements of the posterior embryonic patterning pathway. Our mapping studies place the bicaudal mutation within a approximately 2 kb region, 3′ to the protein coding sequence of the Drosophila homolog of beta NAC, a subunit of Nascent polypeptide Associated Complex (NAC). Genomic DNA encoding beta NAC completely rescues the bicaudal phenotype. The lethal phenotype of Enhancer of Bicaudal, E(Bic), a mutation hypothesized to affect the bicaudal locus, is also completely rescued by the beta NAC locus. We further demonstrate that the E(Bic) mutation is caused by a P element insertion into the transcribed region of the beta NAC gene. NAC is among the first ribosome-associated entities to bind the nascent polypeptide after peptide bond formation. In contrast to other bicaudal-embryo-producing mutations, bicaudal leads to ectopic translation of mRNA for the posterior determinant nanos, without affecting the localization of mRNA for its upstream regulator, oskar, in the embryo. These findings suggest that repression of nanos mRNA translation occurs on the ribosome and involves a role for beta NAC.

Conformational and metal-binding properties of androcam, a testis-specific, calmodulin-related protein from Drosophila

Androcam is a testis-specific protein of Drosophila melanogaster, with 67% sequence identity to calmodulin and four potential EF-hand calcium-binding sites. Spectroscopic monitoring of the thermal unfolding of recombinant calcium-free androcam shows a biphasic process characteristic of a two-domain protein, with the apo-N-domain less stable than the apo-C-domain. The two EF hands of the C-domain of androcam bind calcium cooperatively with 40-fold higher average affinity than the corresponding calmodulin sites. Magnesium competes with calcium binding [Ka(Mg) approximately 3 x 10(3) M(-1)]. Weak calcium binding is also detected at one or more N-domain sites. Compared to apo-calmodulin, apo-androcam has a smaller conformational response to calcium and a lower alpha-helical content over a range of experimental conditions. Unlike calmodulin, a tryptic cleavage site in the N-domain of apo-androcam remains trypsin sensitive in the presence of calcium, suggesting an altered calcium-dependent conformational change in this domain. The affinity of model target peptides for androcam is 10(3)-10(5) times lower than for calmodulin, and interaction of the N-domain of androcam with these peptides is significantly reduced. Thus, androcam shows calcium-induced conformational responses typical of a calcium sensor, but its properties indicate calcium sensitivity and target interactions significantly different from those of calmodulin. From the sequence differences and the altered calcium-binding properties it is likely that androcam differs from calmodulin in the conformation of residues in the second calcium-binding loop. Molecular modeling supports the deduction that there are significant conformational differences in the N-domain of androcam compared to calmodulin, and that these could affect the surface, conferring a different specificity on androcam in target interactions related to testis-specific calcium signaling functions.

Autonomous control of cell and organ size by CHICO, a Drosophila homolog of vertebrate IRS1-4

The control of growth is fundamental to the developing metazoan. Here, we show that CHICO, a Drosophila homolog of vertebrate IRS1-4, plays an essential role in the control of cell size and growth. Animals mutant for chico are less than half the size of wild-type flies, owing to fewer and smaller cells. In mosaic animals, chico homozygous cells grow slower than their heterozygous siblings, show an autonomous reduction in cell size, and form organs of reduced size. Although chico flies are smaller, they show an almost 2-fold increase in lipid levels. The similarities of the growth defects caused by mutations in chico and the insulin receptor gene in Drosophila and by perturbations of the insulin/IGF1 signaling pathway in vertebrates suggest that this pathway plays a conserved role in the regulation of overall growth by controling cell size, cell number, and metabolism.

Calmodulin mediates calcium-dependent activation of the intermediate conductance KCa channel, IKCa1

Small and intermediate conductance Ca2+-activated K+ channels play a crucial role in hyperpolarizing the membrane potential of excitable and nonexcitable cells. These channels are exquisitely sensitive to cytoplasmic Ca2+, yet their protein-coding regions do not contain consensus Ca2+-binding motifs. We investigated the involvement of an accessory protein in the Ca2+-dependent gating of hIKCa1, a human intermediate conductance channel expressed in peripheral tissues. Cal- modulin was found to interact strongly with the cytoplasmic carboxyl (C)-tail of hIKCa1 in a yeast two-hybrid system. Deletion analyses defined a requirement for the first 62 amino acids of the C-tail, and the binding of calmodulin to this region did not require Ca2+. The C-tail of hSKCa3, a human neuronal small conductance channel, also bound calmodulin, whereas that of a voltage-gated K+ channel, mKv1.3, did not. Calmodulin co-precipitated with the channel in cell lines transfected with hIKCa1, but not with mKv1. 3-transfected lines. A mutant calmodulin, defective in Ca2+ sensing but retaining binding to the channel, dramatically reduced current amplitudes when co-expressed with hIKCa1 in mammalian cells. Co-expression with varying amounts of wild-type and mutant calmodulin resulted in a dominant-negative suppression of current, consistent with four calmodulin molecules being associated with the channel. Taken together, our results suggest that Ca2+-calmodulin-induced conformational changes in all four subunits are necessary for the channel to open.

Calmodulin binding to myosin light chain kinase begins at substoichiometric Ca2+ concentrations: a small-angle scattering study of binding and conformational transitions

We have used small-angle scattering to study the calcium dependence of the interactions between calmodulin (CaM) and skeletal muscle myosin light chain kinase (MLCK), as well as the conformations of the complexes that form. Scattering data were measured from equimolar mixtures of a functional MLCK and CaM or a mutated CaM (B12QCaM) incompetent to bind Ca2+ in its N-terminal domain, with increasing Ca2+ concentrations. To evaluate differences between CaM-enzyme versus CaM-peptide interactions, similar Ca2+ titration experiments were performed using synthetic peptides based on the CaM-binding sequence from MLCK (MLCK-I). Our data show there are different determinants for CaM binding the isolated peptide sequence compared to CaM binding to the same sequences within the enzyme. For example, binding of either CaM or B12QCaM to the MLCK-I peptide is observed even in the presence of EGTA, whereas binding of CaM to the enzyme requires Ca2+. The peptide studies also show that the conformational collapse of CaM requires both the N and C domains of CaM to be competent for Ca2+ binding as well as interactions with each end of MLCK-I, and it occurs at approximately 2 mol of Ca2+/mol of CaM. We show that CaM binding to the MLCK enzyme begins at substoichiometric concentrations of Ca2+ (< or = 2 mol of Ca2+/mol of CaM), but that the final compact structure of CaM with the enzyme requires saturating Ca2+. In addition, MLCK enzyme does bind to 2Ca2+ x B12QCaM, although this complex is more extended than the complex with native CaM. Our results support the hypothesis that CaM regulation of MLCK involves an initial binding step at less than saturating Ca2+ concentrations and a subsequent activation step at higher Ca2+ concentrations.

Calcium-binding proteins and development

The known roles for calcium-binding proteins in developmental signaling pathways are reviewed. Current information on the calcium-binding characteristics of three classes of cell-surface developmental signaling proteins (EGF-domain proteins, cadherins and integrins) is presented together with an overview of the intracellular pathways downstream of these surface receptors. The developmental roles delineated to date for the universal intracellular calcium sensor, calmodulin, and its targets, and for calcium-binding regulators of the cytoskeleton are also reviewed.

Increased transmitter release and aberrant synapse morphology in a Drosophila calmodulin mutant

The ubiquitous calcium-binding protein calmodulin (CaM) has been implicated in the development and function of the nervous system in a variety of eukaryotic organisms. We have generated mutations in the single Drosophila Calmodulin (Cam) gene and examined the effects of these mutations on behavior, synaptic transmission at the larval neuromuscular junction, and structure of the larval motor nerve terminal. Flies hemizygous for Cam3c1, a mutation in the first Ca2+-binding site, exhibit behavioral, neurophysiological, and neuroanatomical abnormalities. In particular, adults exhibit defects in locomotion, coordination, and flight. Larvae exhibit increased neurotransmitter release from the motor nerve terminal at low [Ca2+] in the presence of the K+ channel-blocking drug quinidine. In addition, synaptic bouton structure at motor nerve terminals is altered. These effects are distinct from those produced by altering the activity of the CaM target enzymes CaM-activated kinase II (CaMKII) and CaM-activated adenylyl cyclase (CaMAC). Furthermore, previous in vitro studies of mutant Cam3c1 demonstrated that although its Ca2+ affinity is decreased, Cam3c1 protein can activate CaMKII, CaMAC, and CaM-activated phosphatase calcineurin in a manner similar to wild-type CaM. Thus, the Cam3c1 mutation might affect Ca2+ buffering or interfere with the activation or inhibition of a CaM target distinct from CaMKII or CaMAC.

High affinity Ca2+ binding sites of calmodulin are critical for the regulation of myosin Ibeta motor function

We coexpressed myosin Ibeta heavy chain with three different calmodulin mutants in which the two Ca2+-binding sites of the two N-terminal domain (E12Q), C-terminal domain (E34Q), or all four sites (E1234Q) are mutated in order to define the importance of these Ca2+ binding sites to the regulation of myosin Ibeta. The calmodulin mutated at the two Ca2+ binding sites in N-terminal domain and C-terminal domain lost its lower affinity Ca2+ binding site and higher affinity Ca2+ binding site, respectively. We found that, based upon the change in the actin-activated ATPase activities and actin translocating activities, myosin Ibeta with E12Q calmodulin has the regulatory characteristics similar to myosin Ibeta containing wild-type calmodulin, while myosin Ibeta with E34Q or E1234Q calmodulin lose all Ca2+ regulation. While the increase in myosin Ibeta ATPase activity paralleled the dissociation of 1 mol of calmodulin from myosin Ibeta heavy chain for both wild type (above pCa 5) and E12Q calmodulin (above pCa 6), the Ca2+ level required for the inhibition of actin-translocating activity of myosin Ibeta was lower than that required for dissociation of calmodulin, suggesting that the conformational change induced by the binding of Ca2+ at the high affinity site but not the dissociation of calmodulin is critical for the inhibition of the motor activity. Our results suggest that the regulation of unconventional myosins by Ca2+ is directly mediated by the Ca2+ binding to calmodulin, and that the C-terminal pair of Ca2+-binding sites are critical for this regulation.

Calmodulin point mutations affect Drosophila development and behavior

Calmodulin (CAM) is recognized as a major intermediary in intracellular calcium signaling, but as yet little is known of its role in developmental and behavioral processes. We have generated and studied mutations to the endogenous Cam gene of Drosophila melanogaster that change single amino acids within the protein coding region. One of these mutations produces a striking pupal lethal phenotype involving failure of head eversion. Various mutant combinations produce specific patterns of ectopic wing vein formation or melanotic scabs on the cuticle. Anaphase chromosome bridging is also seen as a maternal effect during the early embryonic nuclear divisions. In addition, specific behavioral defects such as poor climbing and flightlessness are detected among these mutants. Comparisons with other Drosophila mutant phenotypes suggests potential CAM targets that may mediate these developmental and behavioral effects, and analysis of the CAM crystal structure suggests the structural consequences of the individual mutations.

Calmodulin regulation of Drosophila light-activated channels and receptor function mediates termination of the light response in vivo

Calmodulin (CAM) participates in a variety of intracellular transduction processes by modulating signaling molecules in response to calcium changes. We report the characterization of Drosophila Cam mutants and the role of CAM in photoreceptor cell function. Contrary to current models of excitation and TRP channel function, we demonstrate that the transient phenotype of trp mutants can be explained by CAM regulation of the TRPL channel rather than by the loss of a store-operated conductance leading to depletion of the internal stores. We also analyzed light responses in a variety of mutant and transgenic backgrounds and demonstrate the importance of calmodulin in mediating calcium-dependent negative regulation of phototransduction. Our results show that CAM coordinates termination of the light response by modulating receptor and ion channel activity.

Calcium binding sites of calmodulin and electron transfer by neuronal nitric oxide synthase

Although binding of calmodulin (CaM) to neuronal nitric oxide synthase (nNOS) has been demonstrated to act as the trigger necessary for electron transfer and catalytic activity, the exact manner in which this is achieved is unclear. By using a series of single point mutants of Drosophila melanogaster CaM, the role that each Ca2+ binding site plays in the transfer of electrons within nNOS has been examined. In these mutants, the bidentate glutamic acid (E) residue which coordinates Ca2+ at the -Z position in each site has been mutated to a glutamine (Q), preventing Ca2+ binding at that site. The results demonstrate that Ca2+ binding at site I of CaM is critical for all electron transfer reactions. All nNOS activities measured (citrulline formation, NADPH oxidation, and cytochrome c reduction) in the presence of the site I CaM mutant (denoted B1Q) were only 2% of the nNOS activity with wild-type CaM. The B2Q and B4Q mutants activated nNOS to similar levels. These two mutants, however, affected nNOS heme-dependent activities to a greater extent than they affected activities independent of the nNOS heme. The site III CaM mutant (B3Q) activated nNOS to levels similar to activities measured with wild-type CaM. Rates of formation of the ferrous-CO complex were also obtained with each of the mutant CaMs. The relative binding affinities of these mutants do not correlate with the observed differences in electron transfer rates. These results demonstrate that, although binding of CaM to nNOS is necessary for catalysis, specific interactions between the two proteins exist which are required for efficient electron transfer.

Characterization of Eu(III) binding to a series of calmodulin binding site mutants using laser-induced Eu(III) luminescence spectroscopy

Laser-induced luminescence techniques were used in a rigorous evaluation of the Eu(3+)-binding behavior of a recombinant (Drosophila melanogaster) calmodulin and a series of calmodulin binding site mutants in which the bidentate glutamic acid residue in position 12 of each metal ion binding loop is systematically replaced with lysine. For the range of Ca2+ concentrations at which calmodulin functions (10(-5)-10(-6) M), Ca2+ binding is effectively eliminated at the mutated site; however, the luminescence studies show that the Eu3+ ion binds to the modified site with reduced affinity. The mutations do not significantly change the intermetal ion distances from their wild type values. These were determined by Eu3+-->Nd3+ Förster-type non-radiative energy transfer experiments. Consistent with the results of Ca(2+)-binding studies, mutation of sites II and IV in the N- and C-terminal domains, respectively, produces a larger alteration in the Eu(3+)-luminescence and Eu(3+)-binding behavior than does mutation of sites I and III. Modification of either of the sites in the C-terminus (III or IV, numbered from the amino terminus) causes two additional H2O molecules (four H2O molecules total) to bind to the Eu3+ ion in order to compensate for the loss of the bidentate glutamic acid residue. Consequently, the partner site in the domain loses an H2O molecule, thereby coordinating another ligand from the protein. Mutation of either of the high-affinity Ln(3+)-binding sites (I or II) has global effects on the Eu(3+)-binding behavior of the protein molecule.

Functional diversity of alternatively spliced isoforms of Drosophila Ca2+/calmodulin-dependent protein kinase II. A role for the variable domain in activation

Isoforms of calcium/calmodulin-dependent protein kinase II from Drosophila (R1-R6 and R3A) showed differential activation by two series of mutant calmodulins, B1K-B4K and B1Q-B4Q. These mutant calmodulins were generated by changing a glutamic acid in each of the four calcium binding sites to either glutamine or lysine, altering their calcium binding properties. All mutations produced activation defects, with the binding site 4 and B1Q mutants the most severe. Activation differed substantially between isoforms. R4, R5, and R6 were the least sensitive to mutations in calmodulin, while R1, R3, and R3A were the most sensitive. Activation of R1 and R2 by B4K and activation of R3 and R3A by B2K and B2Q produced significant (6-fold and almost 3-fold, respectively) differences in Kact between isoforms that differ structurally by a single amino acid. These differences could not be accounted for by differential binding, as all isoforms showed almost identical binding patterns with the mutants. High binding affinity did not always correlate with ability to increase enzyme activity, implying that activation occurs in at least two steps. The isoform-specific differences seen in this study reflect a role for the COOH-terminal variable region in activation of CaM kinase II.

Spontaneous avoidance behavior in Drosophila null for calmodulin expression

The regulatory protein calmodulin is a major mediator of calcium-induced changes in cellular activity. To analyze the roles of calmodulin in an intact animal, we have generated a calmodulin null mutation in Drosophila melanogaster. Maternal calmodulin supports calmodulin null individuals throughout embryogenesis, but they die within 2 days of hatching as first instar larvae. We have detected two pronounced behavioral abnormalities specific to the loss of calmodulin in these larvae. Swinging of the head and anterior body, which occurs in the presence of food, is three times more frequent in the null animals. More strikingly, most locomotion in calmodulin null larvae is spontaneous backward movement. This is in marked contrast to the wild-type situation where backward locomotion is seen only as a stimulus-elicited avoidance response. Our finding of spontaneous avoidance behavior has striking similarities to the enhanced avoidance responses produced by some calmodulin mutations in Paramecium. Thus our results suggest evolutionary conservation of a role for calmodulin in membrane excitability and linked behavioral responses.

Interlobe communication in multiple calcium-binding site mutants of Drosophila calmodulin

We have generated mutants of Drosophila calmodulin in which pairs of calcium-binding sites are mutated so as to prevent calcium binding. In all sites, the mutation involves replacement of the -Z position glutamate residue with glutamine. Mutants inactivated in both N-terminal sites (B12Q) or both C-terminal sites (B34Q), and two mutants with one N- and one C-terminal site inactivated (B13Q and B24Q) were generated. The quadruple mutant with all four sites mutated was also studied. UV-difference spectroscopy and near-UV CD were used to examine the influence of these mutations upon the single tyrosine (Tyr-138) of the protein. These studies uncovered four situations in which Tyr-138 in the C-terminal lobe responds to a change to the calcium-binding properties of the N-terminal lobe. Further, they suggest that N-terminal calcium-binding events contribute strongly to the aberrant behavior of Tyr-138 seen in mutants with a single functional C-terminal calcium-binding site. The data also indicate that loss of calcium binding at site 1 adjusts the aberrant conformation of Tyr-138 produced by mutation of site 3 toward the wild-type structure. However, activation studies for skeletal muscle myosin light chain kinase (SK-MLCK) established that all of the multiple binding site mutants are poor activators of SK-MLCK. Thus, globally, the calcium-induced conformation of B13Q is not closer to wild type than that of either the site 1 or the site 3 mutant. The positioning of Tyr-138 within the crystal structure of calmodulin suggests that effects of the N-terminal lobe on this residue may be mediated via changes to the central linker region of the protein.

Recovery of native structure by calcium binding site mutants of calmodulin upon binding of sk-MLCK target peptides

The calcium-dependent binding of two synthetic 18-residue peptides derived from the calmodulin binding region of skeletal myosin light chain kinase to wild-type Drosophila melanogaster calmodulin and four calcium binding site calmodulin mutants has been investigated using optical spectroscopy. The WFF peptide (with W4 and F17) and the FFW peptide (with F4 and W17) both bind to wild-type calmodulin with 1:1 stoichiometry and Kd values of < or = 0.2 and 1.6 nM, respectively. Near-UV CD spectra of the protein-peptide complexes suggest that both peptides bind in the same orientation, with the side chain of residue 4 interacting with the C-domain of calmodulin and that of residue 17 with the N-domain [as in the structure of the calmodulin-M13 peptide complex determined by Ikura et al. [Ikura, M., Clore, G. M., Gronenborn, A. M., Zhu, G., Klee, C. B., & Bax, A. (1992) Science 256, 632-638]]. Both peptides have lower affinities for all the mutant calmodulins than for the wild-type protein. Fluorescence measurements suggest that mutation of calcium binding site 2 in the N-domain does not affect the interaction of the W4 side chain of the WFF peptide with the C-domain of calmodulin. However, the E67Q (B2Q) but not the E67K (B2K) mutation (site 2, N-domain) alters the interaction of W17 of the FFW peptide with the protein. In contrast, the E140K (B4K) mutation has a much greater effect than the E140Q (B4Q) mutation (site 4, C-domain) on the interaction of calmodulin with both peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of four enzymes by two series of calmodulin mutants with point mutations in individual Ca2+ binding sites

Activation of four target enzymes by two series of calmodulin Ca2+ binding site mutants has been examined. In each mutant, the conserved bidentate glutamate of one of the Ca2+ binding sites is mutated to glutamine or lysine. The enzymes studied were smooth and skeletal muscle myosin light chain kinases, adenylylcyclase, and plasma membrane Ca(2+)-ATPase. For the first three enzymes, the activation patterns with the two mutant series were very similar: mutation of site 4 was most deleterious, then site 2, site 3, and site 1. This ranking was observed previously in Ca2+ binding and Ca(2+)-induced conformational studies of these mutants. Thus the response of these enzymes is probably determined by the extent to which each mutant's competence to interact with target binding regions has been compromised. In contrast, for Ca(2+)-ATPase, mutants of sites 3 and 4 were much poorer activators than those of sites 1 and 2. Events beyond calmodulin binding and related to enzyme activation probably dictate this unusual activation pattern and also the anomalously poor activation of skeletal muscle myosin light chain kinase by site 1 mutant B1Q. Site 1 mutant B1K showed wild type activation of all four enzymes suggesting that in site 1, the lysine substitution can evoke the conformational changes associated with Ca2+ binding.

A series of point mutations reveal interactions between the calcium-binding sites of calmodulin

Calmodulin is a member of the "EF-hand" family of Ca(2+)-binding proteins. It consists of two homologous globular domains, each containing two helix-loop-helix Ca(2+)-binding sites. To examine the contribution of individual Ca(2+)-binding sites to the Ca(2+)-binding properties of CaM, a series of four site-directed mutants has been studied. In each, the glutamic acid at position 12 in one of the four Ca(2+)-binding loops has been changed to a glutamine. One-dimensional 1H-NMR has been used to monitor Ca(2+)-induced changes in the mutant proteins, and the spectral changes observed for each mutant have been compared to those for wild-type CaM. In this way, the effect of each mutation on both the mutated site and the other Ca(2+)-binding sites has been examined. The mutation of glutamate to glutamine at position 12 in any of the EF-hand Ca(2+)-binding loops greatly decreases the Ca(2+)-binding affinity at that site, yet differs in the overall effects on Ca2+ binding depending on which of the four sites is mutated. When the mutation is in site I, there is only a small decrease in the apparent Ca(2+)-binding affinity of site II, and vice versa. Mutation in either site III or IV results in a large decrease in the apparent Ca(2+)-binding affinities of the partner C-terminal site. In both the N- and C-terminal domains, evidence for altered conformational effects in the partners of mutated sites is presented. In the C-terminus, the conformational consequences of mutating site III or site IV are strikingly different.

Calcium binding site mutants of calmodulin adopt abnormal conformations in complexes with model target peptides

We have examined the ability of two series of calcium binding site mutants of Drosophila calmodulin to form complexes with the model target peptides melittin and mastoparan. Unlike the wild-type complex, the 1:1 protein:melittin complexes formed by mutants of the C-terminal sites are unable to bind a second molecule of melittin. In contrast, a site 2 mutant shows increased ability to bind two molecules of melittin. For the shorter peptide mastoparan, most mutants form aberrant complexes that are best interpreted in terms of a model in which mastoparan interacts with both terminal domains of calmodulin. For two of the target enzymes of calmodulin, the three mutants which form mastoparan complexes most similar to the wild-type complex are also the best enzyme activators.

Circular dichroism studies on calcium binding to two series of Ca2+ binding site mutants of Drosophila melanogaster calmodulin

The Ca(2+)-induced structural changes in mutant calmodulins from Drosophila melanogaster have been studied by circular dichroism. The proteins comprise eight site-specific mutants, in which a bidentate glutamic acid (at position 12 in each Ca2+ binding loop) is replaced with either glutamine (BQ series) or lysine (BK series). Previous studies of these proteins indicate that Ca2+ binding at the mutated site is effectively eliminated by each of these substitutions, with additional effects at nonmutated sites. Circular dichroism has now been used to assess Ca(2+)-induced changes in secondary and tertiary structure in these proteins. In the absence of Ca2+, the helical content of these mutant calmodulins is close to that of the wild-type protein. In excess Ca2+, calmodulins with a mutation in the N-terminal sites show Ca(2+)-induced increases in helicity (CD at 222 nm) that are similar to those of the wild-type protein. In contrast, much less additional helix is induced by Ca2+ in calmodulins with mutations in the C-terminal sites, with the two mutations to site IV showing a particularly poor response. Ca(2+)-induced changes to the environment of the single tyrosine of Drosophila calmodulin (Tyr-138 in site IV of the C-terminal domain) have been monitored via CD at 280 nm. The signal from this residue is significantly altered in the Ca(2+)-free form of almost all these mutants, including those in the N-terminal domain. This indicates significant interaction between the N- and C-terminal domains of these mutants.(ABSTRACT TRUNCATED AT 250 WORDS)

Stopped-flow studies of calcium dissociation from calcium-binding-site mutants of Drosophila melanogaster calmodulin

The kinetics of calcium dissociation from two groups of site-specific mutants of calmodulin from Drosophila melanogaster have been studied by stopped-flow kinetic methods, using the fluorescent calcium chelator 8-amino-2-[(2-amino-5-methylphenoxy)methyl]-6- methoxyquinoline-N,N,N',N'-tetraacetic acid (Quin 2). The BQ series of mutants consists of four proteins in which one of the four bidentate glutamate residues (Glu12 of each of the four calcium binding loops) has been replaced by glutamine. In the BK series of mutants, the corresponding glutamate has been replaced by lysine. Calcium-dissociation kinetics of proteins with a mutation in site I or II (N-terminal domain) are consistent with a model in which the mutation weakens binding at the non-mutated N-terminal partner site and has a small, but significant, effect on the kinetic properties of sites III and IV (C-terminal domain). The proteins with a mutation in site III or IV show a large effect, with decreased Ca2+ dissociation rate from the unmodified N-terminal Ca(2+)-binding sites I and II. A structural interpretation is proposed, based on enhanced interactions between the domains when the affinity of individual sites have been dramatically reduced by mutation. This effect is greatest for the mutations in the C-terminal domain, which appear to destroy the co-operativity of Ca2+ binding at sites III and IV. The results show that site-specific mutation can have surprisingly far-ranging effects on kinetic properties of calmodulin. The kinetic analysis also shows that studies of specifically engineered mutants may in principle help to unmask the values of intrinsic rate constants for the wild-type protein which are not normally observable in the process of Ca2+ dissociation.

Ca2+ binding and conformational change in two series of point mutations to the individual Ca(2+)-binding sites of calmodulin

Two series of site-directed mutations to the individual Ca(2+)-binding sites of Drosophila melanogaster calmodulin have been generated and studied. In each mutant, a conserved glutamic acid residue at position 12 in all of the Ca(2+)-binding loops has been mutated in one site. In one series the residue is changed to glutamine; in the second series the change is to lysine. The Ca(2+)-binding properties of these mutants and the wild-type protein under pseudo-physiological conditions are presented. In addition, Ca(2+)-induced changes to the environment of the single tyrosine residue (Tyr-138) have been studied for some of the mutants. Ca2+ binding to the wild-type protein is best modeled as two pairs of sites with a higher affinity pair that shows strong cooperativity. For all but one of these eight mutant proteins, only three Ca(2+)-binding events can be detected. In three of the amino-terminal mutants, the three residual sites are (i) a pair of relatively high affinity sites and (ii) a weakened low affinity site. For all four carboxyl-terminal mutations, the residual sites are three relatively low affinity sites. In general, mutations to sites 2 and 4 prove more deleterious than mutations to sites 1 and 3. The Ca(2+)-induced conformational changes in the vicinity of Tyr-138 are relatively undisturbed by mutations of site 1. However, the changes to Tyr-138 in the carboxyl-terminal site mutants indicate that upon disruption of the cooperative binding at the high affinity sites, conformational change in the carboxyl terminus occurs in two phases. It appears that binding of Ca2+ to either carboxyl-terminal site can elicit the first phase of the response but the second phase is almost abolished when site 4 is the mutated site. The final conformations of site 3 and 4 mutants are thus significantly different.

Calmodulin transcription is limited to the nervous system during Drosophila embryogenesis

We have examined the RNA expression pattern for the Drosophila calmodulin gene during embryogenesis by in situ hybridization to transcripts in whole embryos. Our results indicate that maternally derived calmodulin mRNA is homogeneously distributed throughout the early embryo, but that these maternal transcripts are lost by maximal germ band extension. Zygotic transcription of the gene in mid- to late-stage embryos is restricted to neural cell precursors and their progeny in both the central and peripheral nervous systems. Thus, activation of calmodulin transcription during embryonic development appears to mark a commitment to a neural fate. Northern blot analysis revealed that the two transcripts from the calmodulin gene are differentially expressed during embryogenesis. Comparison of Northern blot and in situ hybridization data indicates that the longer calmodulin mRNA is a nervous tissue-specific transcript. This suggests that neural-specific regulation of polyadenylation site usage occurs. We have also examined calmodulin expression in embryos homozygous for mutations in four loci which are known to affect nervous system development: numb, the achaete-scute complex, daughterless, and mastermind. The calmodulin transcription pattern is altered in embryos mutant for each of these loci, suggesting that regulation by these genes, either directly or indirectly, is taking place.

Structure of a recombinant calmodulin from Drosophila melanogaster refined at 2.2-A resolution

The crystal structure of calmodulin (Mr 16,700, 148 residues) from Drosophila melanogaster as expressed in a bacterial system has been determined and refined at 2.2-A resolution. Starting with the structure of mammalian calmodulin, we produced an extensively refitted and refined model with a conventional crystallographic R value of 0.197 for the 5,239 reflections (F greater than or equal to 2 sigma (F)) within the 10.0-2.2-A resolution range. The model includes 1,164 protein atoms, 4 calcium ions, and 78 water molecules and has root mean square deviations from standard values of 0.018 A for bond lengths and 0.043 A for angle distances. The overall structure is similar to mammalian calmodulin, with a seven-turn central helix connecting the two calcium-binding domains. The "dumb-bell" shaped molecule contains seven alpha-helices and four "EF hand" calcium-binding sites. Although the amino acid sequences of mammalian and Drosophila calmodulins differ by only three conservative amino acid changes, the refined model reveals a number of significant differences between the two structures. Superimposition of the structures yields a root mean square deviation of 1.22 A for the 1,120 equivalent atoms. The calcium-binding domains have a root mean square deviation of 0.85 A for the 353 equivalent atoms. There are also differences in the amino terminus, the bend of the central alpha-helix, and the orientations of some of the side chains.

Use of site-directed mutations in the individual Ca2(+)-binding sites of calmodulin to examine Ca2(+)-induced conformational changes

Mutant versions of the calmodulin of Drosophila melanogaster have been prepared for use in the study of Ca2+ binding and Ca2(+)-induced conformational changes. In each mutant, a conserved glutamic acid residue indicated to play a critical role in Ca2+ binding has been mutated to glutamine in one of the Ca2(+)-binding sites. Thus a series of four proteins, each with an analogous mutation in one of the four binding sites, has been generated. Here the Ca2(+)-induced conformational changes in these proteins have been examined by use of the fluorescent hydrophobic reporter molecule, 9-anthroyl choline. These studies confirm earlier work which indicates that the carboxyl-terminal pair of Ca2(+)-binding sites shows cooperative Ca2+ binding to produce a major conformational change in the protein. However, these studies provide evidence that the sites of the amino-terminal pair are more independent in their Ca2+ binding properties and contribute individually to the conformational changes associated with Ca2+ binding in the amino-terminal half of the protein. This work also indicates that mutation of either of the amino-terminal Ca2(+)-binding sites can influence the conformational change produced by Ca2+ binding to the carboxyl-terminal sites.

A second maternally expressed Drosophila gene encodes a putative RNA helicase of the "DEAD box" family

Recently, a family of proteins containing the conserved motif Asp-Glu-Ala-Asp, the "DEAD box" proteins, has been identified. This family is typified by the eukaryotic translation initiation factor eIF4A, and its members are believed to share the functional property of ATP-dependent RNA unwinding. One of the previously identified members of this family (vasa) is the product of a maternally expressed gene from Drosophila melanogaster that is known to play a role in the formation of the embryonic body plan. We report here the isolation of a Drosophila gene that has an mRNA expression pattern somewhat similar to that of vasa and also encodes a DEAD box protein. We have termed this gene ME31B to reflect its maternal (ovarian germ-line) expression and its location within the 31B chromosome region. Comparisons with the other members of this family reveal that although ME31B is most like the protein Tif1/Tif2, which probably represents the Saccharomyces cerevisiae version of eIF4A, it is unlikely that ME31B represents the Drosophila eIF4A protein per se. A search for mutations in the ME31B gene has established that the P element which causes the female-sterile mutation flipper lies in the 3' flank of the ME31B gene.

Drosophila melanogaster contains a single calmodulin gene. Further structure and expression studies

We have previously characterized a calmodulin gene from the organism Drosophila melanogaster. In the higher vertebrates a multi-gene system for encoding calmodulin is present and, in at least one invertebrate species, genes encoding highly related calmodulin isotypes exist. We have therefore searched for additional calmodulin genes within D. melanogaster. Although our searches were sensitive enough to detect a relatively divergent gene encoding a calmodulin family protein, we were unable to detect any additional genes for calmodulin per se. Further studies of the structure and expression of the single calmodulin gene of D. melanogaster have established that the gene contains a tiny additional 5' exon encoding only 50 residues of the 5' leader. Sequencing at the 3' terminus has established that the two transcript size classes derived from the gene are produced as a result of alternative polyadenylation site usage. The relative abundance of the two size classes of mRNAs differs throughout the life cycle, indicating developmental regulation of polyadenylation site usage.

Genes with specific functions in the ovarian follicles of Calliphora erythrocephala (Diptera)

Working with the large dipteran Calliphora erythrocephala, we have performed differential screening to identify genes actively expressed in the previtellogenic and early vitellogenic stages of oogenesis but silent during the early stages of embryogenesis. Clones containing sequences homologous to four such genes have been characterized. Two clones are homologous to the yolk protein 1 gene of Drosophila melanogaster. These two clones are expressed not only in the columnar follicle cells surrounding the oocyte but also in the border cells--a highly specialized subgroup of the follicle cells. This indicates a new function for these cells though previously to contribute mainly in the formation of the micropyle. A third clone, which is related to the D. melanogaster vitelline membrane protein genes of the cluster at chromosomal locus 26A, is expressed only in the perioocyte follicle cells and not the border cell population. The fourth clone encodes a sequence of unknown function which is abundantly expressed in the germ line cells of the follicle. Transcripts homologous to this clone persist into the mature follicle and initially appear concentrated at the anterior pole of the oocyte. The distribution of repetitious DNA within these four clones indicates that the C. erythrocephala genome has a short interspersion arrangement of repetitive DNA.

Structure and sequence of the Drosophila melanogaster calmodulin gene

A series of phage clones overlapping the single calmodulin gene locus of Drosophila melanogaster has been isolated and the exons of the gene positioned and sequenced within these clones. A calmodulin cDNA clone of the electric eel was used to identify these clones and to position the two major protein-coding exons of the gene. cDNA clones for D. melanogaster calmodulin were then isolated, characterized and used to identify the remaining exons. The gene consists of four exons separated by three introns of 3400 to 4300 bases in length. Exon 1 consists of the 5' untranslated region and the initiator ATG; exon 2 encodes amino acid residues 1 to 58.3; exon 3 encodes residues 58.3 to 139.3; and exon 4 encodes residues 139.3 to 148 and the 3' untranslated region. From the sequence of the 3' untranslated region and the lengths of the cDNA clones, two or three polyadenylation sites are indicated. Sequences potentially involved in the control of transcription of the gene and splicing of the mRNA product have been identified. Comparison of the intron-exon structures of the D. melanogaster calmodulin gene, the chick calmodulin gene, and other genes of the troponin C superfamily reinforces previous hypotheses that these genes arose from a common progenitor and permits identification of four introns that were probably present in the progenitor gene structure. The D. melanogaster calmodulin gene contains three of these introns, and the chick gene contains all four. These gene comparisons also indicate that the region of these genes encoding Ca2+-binding loop 3 is highly variable in structure. The chick and D. melanogaster calmodulin genes differ in this region, the chick gene containing a fifth intron here that is absent from the D. melanogaster gene.

Isolation and characterization of abl gene sequences in Calliphora erythrocephala

Screening of genomic DNA libraries with hybridization probes derived from a Drosophila melanogaster c-abl proto-oncogene homologue resulted in the isolation of a set of related sequences from the dipteran Calliphora erythrocephala. Although the region encompassing the c-abl protein kinase domain encodes a polypeptide extremely similar to the Drosophila gene, considerable inter- and intraspecific divergence is found adjacent to this region. Restriction-site heterogeneity and cross-hybridization studies between individual cloned isolates suggest that abl homologues represent a small gene family in the Calliphora genome. As is the case in Drosophila, abl-related transcripts appear to be low in abundance, are synthesized during oogenesis and stored as a maternal mRNA.

3B55: a repetitious sequence family which is transcribed and proportionately replicated in germ-line polyploid nuclei of Calliphora erythrocephala

The chromosomes of dipteran polyploid nurse cell nuclei are functionally analogous to the oocyte lampbrush chromosomes of the Amphibia. In investigating the transcriptional and replicative activity of these nuclei in Calliphora erythrocephala we have identified a cloned highly repetitious DNA fragment which shows enhanced transcriptional activity in these cells and different replicative behavior in these germ-line polyploid nuclei as opposed to somatic polytene nuclei. The clone, 3B55, contains a 6.8-kb insert which consists primarily of tandemly repeated 200-bp sequences defined by RsaI sites. From Southern hybridizations to diploid (embryonic) genomic DNA, 3B55-related DNA was calculated to represent a significant fraction of the haploid genome (0.8% or 5000 kb). In situ hybridizations established that these sequences are present in the pericentric regions of four of the six chromosomes. Thus the 3B55 sequences have the properties of a satellite-type DNA family. Quantitation of the 3B55 200-bp monomer (which represents approximately 60% of the genomic 3B55 DNA sequences in all tissues examined) revealed that in somatic polytene salivary gland nuclei, 3B55 DNA is highly under-replicated to give a monomer representation of only 48 +/- 11 kb per haploid genome. However, in germ-line nurse cell nuclei, 3B55 DNA is proportionately replicated to give a monomer genomic representation (3560 +/- 344 kb) equivalent to that of diploid DNA (3011 +/- 202 kb). Transcripts complementary to 3B55 sequences are at least 25 times more abundant in total nurse cell nuclear RNA than in total embryonic nuclear RNA. These findings suggest an association of the 3B55 sequence family with some germ-line specific function.

Both nucleolar organizers are replicated in Dipteran polyploid tissues: a study at the level of individual nuclei

Working with the Dipteran Calliphora erythrocephala, we have tested the hypothesis that only one nucleolar organizer region (NO) is replicated during polyploidization. NO replication was examined in two very different highly polyploid nuclear types: salivary gland nuclei and nurse cell nuclei. Two strains of the organism containing NO regions with highly diagnostic nontranscribed spacer (NTS) polymorphisms were prepared and reciprocal single pair-matings between members of the strains were performed. The representation of the two distinguishable NOs in diploid and polyploid DNAs of individual F1 progeny from each cross was then examined. DNA from a total polyploid nuclear DNA preparation and from individual polyploid nuclei of both tissue types was analyzed. Our results show conclusively that both genomic NOs are replicated in individual polyploid nuclei of both types. Further, evidence for variation in the relative replication of cistrons from the two NOs by individual nuclei was obtained. The cistron types present in the NOs of both strains showed differential replication upon polyploidization. In general, the patterns of differential cistron replication seen in salivary gland and nurse cell nuclei were similar.

Conserved 5' flank homologies in dipteran 5S RNA genes that would function on 'A' form DNA

We have sequenced the 480 base pair (bp) repeating unit of the 5S RNA genes of the Dipteran fly Calliphora erythrocephala and compared this sequence to the three known 5S RNA gene sequences from the Dipteran Genus Drosophila (1,2). A striking series of five perfectly conserved homologies identically positioned within the 5' flanks of all four Dipteran 5S RNA coding regions has thus been identified. The spacing (12-13 bp) between all of these homologies is typical of A form rather than B form DNA. Given that the eukaryotic 5S RNA gene specific initiation factor TFIIIA (3) is a DNA unwinding protein (4), a role for these Dipteran 5' flank homologies in initiation site selection on 5S RNA genes transiently unwound for transcription is suggested. One of the Dipteran homology blocks is highly conserved in sequence and position in all but one of the eukaryotic 5S RNA gene sequences known to date (17/18 genes). Its sequence (consensus: TATAAG) and position (average center: -26 bp) are highly reminiscent of the polymerase II gene 'TATA' box (5).

Different chromatin states of the intron- and type 1 intron+ rRNA genes of Calliphora erythrocephala

In most species of dipteran fly examined, a fraction of the rDNA cistrons are interrupted by introns. These dipteran intron+ rRNA genes are unique in that they are transcriptionally inactive. Previous studies have investigated the mechanism underlying this transcriptional repression for rRNA genes carrying the best characterized sequence family of such introns, the so-called type 1 introns first identified in Drosophila melanogaster. These studies have established that cloned examples of both intron-free and type 1 intron+ rRNA genes will support transcription in a cell-free system and suggest therefore that a difference in the chromatin state of the two gene types must underlie their very different potential for in vivo transcription. We have examined this possibility for the type 1 intron+ rDNA cistrons of Calliphora erythrocephala by in situ hybridization studies using the polytene chromosome complement of the pupal bristle-forming (trichogen) cells. These studies show that the chromatin configuration of the two gene types is strikingly different. The intron-free genes are preferentially localized in the actively transcribed fibrillar center of the nucleolus. The intron+ genes are preferentially condensed in the blocks of heterochromatin attached to the nucleolus.

The intron boundaries and flanking rRNA coding sequences of Calliphora erythrocephala rDNA

We have sequenced the available cloned examples of the intron-coding sequence junctions for the rDNA of the higher Dipteran, Calliphora erythrocephala. The introns interrupt the rDNA at the same position as the type 1 intron family detected in Drosophila melanogaster and D. virilis (10,11). A duplication of 14 base pairs of the 28S rRNA coding sequence surrounds a short version of the major genomic length class of introns. This same duplication is associated with boundaries of the type 1 introns in D. virilis and D. melanogaster (10, 13,14). We have detected considerable homology between the 3' intron sequences of C. erythrocephala and D. virilis. The rRNA coding sequences flanking the introns are extremely homologous in C. erythrocephala, D. melanogaster and D. virilis, with only one small region of significant divergence. This corresponds to a variable stem region previously identified in eukaryotic 28S rRNA at a site analogous to the L1 ribosomal protein binding site of prokaryotic 23S rRNA (27).