Sequential regulation of developmental events during polar morphogenesis in Caulobacter crescentus: assembly of pili on swarmer cells requires cell separation

Pili, along with the flagellum and DNA bacteriophage receptors, are structural markers for polar morphogenesis in Caulobacter crescentus. Pili act as primary receptors for a number of small, C. crescentus-specific DNA and RNA bacteriophages, and the timing of pilus-dependent adsorption of bacteriophage phiCb5 in synchronized cell populations has led to the general conclusion that pili are formed coordinately with the flagellum and other polar surface structures in the predivisional cell. The use of rotary platinum shadow casting and electron microscopy as a direct assay for formation of flagella and pili in synchronous cell cultures now shows, however, that when expressed as fractions of the swarmer cell cycle, flagella are assembled on the predivisional cells at approximately 0.8 and that pili are assembled on the new swarmer cells at approximately 0.1 of the next cell cycle. Adsorption of pilus-specific bacteriophage phiCb5 prevented the loss of pili from swarmer cells during development, which suggests that these structures are retracted at the time of stalk formation. Examination of temperature-sensitive cell division mutants showed that the assembly of pili depends on completion of cell separation. These results indicate that the stage-specific events required for polar morphogenesis in C. crescentus occur sequentially, rather than coordinately in the cell cycle, and that the timing of these events reflects the order of underlying cell cycle steps.

Order of gene replication in Caulobacter crescentus; use of in vivo labeled genomic DNA as a probe

Two methods for determining the time of gene replication in Caulobacter crescentus using a temperature sensitive DNA synthesis mutant to synchronize chromosome replication are described. Swarmer cells, blocked before DNA initiation at 37 degrees C, initiate chromosome replication within 2 min after releasing the temperature block in 32P-orthophosphate medium, as indicated by the appearance of a small number of unique genomic DNA fragments. The time at which a given chromosome segment replicates was determined by isolating genomic DNA from cells labeled for progressively longer times during the S period of the cell cycle and hybridizing the probes to cloned C. crescentus genes. The time of replication of genetically mapped Tn5 insertions was determined by preparing DNA from the Tn5 insertion mutants that had been labeled with 32P in similar experiments and hybridizing it to lambda::Tn5 DNA. These results furnish the first correlation between the order of chromosome replication and the genetic map of C. crescentus. They also show that the times of replication and expression of the hook protein and the flagellin genes, which require DNA synthesis for their transcription, both occur near mid-S phase.

Social adjustment and interaction after severe head injury: II. Rationale and bases for intervention

Social anxiety, social performance and self-esteem are suggested to be important factors in the social adjustment of severely head-injured patients. The present study attempted to remediate some of these deficits in social interaction and adjustment by adopting theoretical models of the development of self-understanding, and of the general process of recovery from head injury, together with the application of existing methodology from social skills training. Reassessment after one year of group treatment did not show any statistically significant results, but some important individual changes in social performance and social anxiety were found. Reasons for the apparent failure to change are discussed, with recommendations for more intensive treatment and refinement of measures.

A set of positively regulated flagellar gene promoters in Caulobacter crescentus with sequence homology to the nif gene promoters of Klebsiella pneumoniae

The study reported here describes nuclease S1 mapping of the in-vivo transcription start sites of transcription units I and III of the hook gene cluster of Caulobacter crescentus. We show that transcription units I and II of this flagellar (fla) gene cluster, which have divergent promoters with transcription start sites separated by 218 nucleotides, are under positive transcriptional control by genes in transcription unit III. The promoters of transcription units I, II, and III were compared with flagellin gene promoters P25, P27 and P29 recently identified in C. crescentus. Promoters PII, P25, and P27, which are under positive regulation by transcription units III to V have strongly conserved sequence elements at -13 and -24 with the consensus sequence (C/T)TGGC(C/G)C-N5-TTGC. The -13, -24 sequence elements are not well conserved in promoter PI, but the promoter does contain a copy of the -13 and -24 consensus sequence 23 base-pairs upstream (PI). The C. crescentus fla gene promoters are not homologous to the canonical Escherichia coli -10, -35 promoter sequence, but they are very similar to the -12, -24 nif gene promoter sequence reported for Klebsiella pneumoniae and Rhizobium sp. The four positively regulated fla gene promoters examined here also share a third conserved element designated II-1, with the consensus sequence C-C-CGGC--AAA--GC-G, located at approximately -100. We speculate that the conserved sequence elements mapping at -13, -24 and -100 are cis-acting regulatory elements required for the transcription and periodic regulation of these fla genes in the C. crescentus cell cycle.

Comparison of S100b protein with calmodulin: interactions with melittin and microtubule-associated tau proteins and inhibition of phosphorylation of tau proteins by protein kinase C

To gauge similarities between S100b protein and calmodulin, interactions were observed between S100b and melittin and between S100b and tau, the microtubule-associated proteins. The interaction of melittin with S100b protein in the presence and absence of calcium was studied by fluorescence polarization, UV difference spectroscopy, and sulfhydryl derivatization. Whether calcium was present or not in the solution, melittin and S100b form a complex of molar ratios up to 2:1. Further binding of melittin occurred, but it resulted in precipitation of S100b, as is true of the corresponding case of melittin binding to calmodulin. In the absence of calcium, the interaction of melittin and S100b shielded the tryptophan (Trp) of the former protein and exposed cysteine-84 beta (Cys-84 beta) of the latter protein, leaving the tyrosine-16 beta (Tyr-16 beta) of S100b unaffected. Calcium addition to the complex partially restored the exposure of Trp of melittin and caused changes in the environment of Tyr-16 beta (unlike the environmental changes induced for Tyr-16 beta by calcium in the absence of melittin). The conformational changes induced in S100b by interaction with melittin increased its affinity for calcium and offset the inhibition of calcium binding otherwise observed in the presence of potassium ions. This corroborated the previous finding that S100b affinity for calcium greatly depends on the protein conformation. The phenomena described above are similar to the interactions of melittin with calmodulin and thus suggest that S100b and calmodulin have a common structural domain not only that binds melittin but also that may interact with common target proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Promoter mapping and cell cycle regulation of flagellin gene transcription in Caulobacter crescentus

Caulobacter crescentus contains a 25- and a 27-kDa flagellin, which are assembled into the flagellar filament, and a 29-kDa flagellin, which is related in sequence but is of unknown function. We have used DNA sequence analysis and nuclease S1 assays to map the in vivo transcription start sites of the three flagellin genes and to study their regulation. These experiments lead to several conclusions. First, copies of the 29-, 25-, and 27-kDa flagellin genes are organized in a tandem array in the flaEY gene cluster of C. crescentus. Second, flagellin genes are under transcriptional control and each gene is expressed with a characteristic periodicity in the cell cycle. Third, flagellin gene promoters contain conserved nucleotide sequence elements at -13, -24, and -100 that are homologous to the fla genes in the hook gene cluster. The -13 and -24 sequences conform to a fla gene promoter consensus sequence (C/TTGGCC/GC-N5-TTGC) that is similar in sequence to the -12, -24 consensus sequence of the Klebsiella pneumonia nif gene promoters. Fourth, the sequence element at approximately -100 in the 25- and the 27-kDa flagellin genes is homologous to a 19-base-pair sequence [designated previously as II-1; see Chen, L.-S., Mullin, D. M. & Newton, A. (1986) Proc. Natl. Acad. Sci. USA 83, 2860-2864]at -101 in the promoter of transcription unit II of the hook gene cluster; the two flagellin genes, like the fla genes examined in the hook gene cluster that contain the -100 element, are under positive control by transcription unit III of the hook gene cluster. This result supports a model in which the timing of fla gene transcription in the C. crescentus cell cycle is determined in part by a cascade of trans-acting regulatory gene products.

Differential localization of membrane receptor chemotaxis proteins in the Caulobacter predivisional cell

The methyl-accepting chemotaxis proteins (MCPs) are membrane receptors that initiate signal transduction to the flagellar rotor upon ligand binding. The synthesis of these proteins occurs only in the Caulobacter crescentus predivisional cell coincident with the biosynthesis of the polar flagellum. Both the flagellum and the MCPs are partitioned to only one daughter cell, the swarmer cell, upon division. We report the results of experiments designed to determine the distribution of these MCPs within swarmer cells and predivisional cells. Flagellated and non-flagellated vesicles were prepared from these cells by immunoaffinity chromatography and the level of MCPs that had been labeled either in vivo or in vitro with methyl-3H was determined. Small membrane vesicles from swarmer cells contained [methyl-3H]MCPs both in the flagellated and non-flagellated vesicles, which indicates that the region immediately surrounding the flagellum, as well as the rest of the surface of the swarmer cell, contains [methyl-3H]MCP. Thus, the MCPs are not specifically localized to the immediate vicinity of the flagellar rotor. The distribution of MCPs was examined in flagellated and non-flagellated vesicles isolated from predivisional cells. The analysis of small predivisional vesicles showed that the MCP content is higher in the flagellated vesicles, and analysis of large flagellated vesicles showed that the MCPs are positioned preferentially in the swarmer cell portion of the predivisional cell. This positional bias of MCPs within predivisional cells could reflect either a large compartment or membrane domain within the incipient swarmer cell, or a gradient of MCPs, with the highest concentration in the vicinity of the flagellum.

Identification, nucleotide sequence, and control of developmentally regulated promoters in the hook operon region of Caulobacter crescentus

The major flagellar proteins, including the flagellins and the hook protein, are synthesized periodically in the Caulobacter crescentus cell cycle at the time of flagellum assembly. Although fla genes are regulated at the transcriptional level [Ohta, N., Chen, L.-S., Swanson, E. & Newton, A. (1985) J. Mol. Biol. 186, 107-115], the 5' regulatory regions of C. crescentus genes have not been identified. We describe here the results of nuclease S1 protection assays that map the 5' ends of mRNAs synthesized in vivo from transcription units II (hook operon) and II.1 of the hook gene cluster and locate the corresponding promoter regions PII and PII.1. The two promoters are regulated with different periodicities in the cell cycle and have different genetic requirements for expression. The failure to detect transcripts from either PI or PII in Escherichia coli suggests that developmentally regulated promoters of C. crescentus have different recognition sequences from those of E. coli. There is little nucleotide sequence homology between PII and PII.1. There are, however, three regions of homology between PII and the nucleotide sequence 5' to the 29-kDa-flagellin-related gene, and two of these are in regions of dyad symmetry. We discuss the possibility that DNA-protein interactions at homologous nucleotide sequences like those identified in PII are part of a regulatory gene cascade that participates in timing fla gene expression in the C. crescentus cell cycle.

Transcriptional regulation of a periodically controlled flagellar gene operon in Caulobacter crescentus

Temporal regulation of flagellar gene expression in Caulobacter crescentus has been examined by a detailed analysis of the flbG-flaJ-flbH-flaK hook operon. The approximate location of the promoter for this 4.4 X 10(3) base-pair transcriptional unit was determined by deletion mapping, and the flaK gene was shown by nucleotide sequencing to code for the hook protein. flaK messenger RNA was quantified by S1 nuclease mapping with an internal restriction fragment of the gene as the 5'-labeled DNA probe. The results of these assays provide the first direct evidence that periodic expression of a flagellar gene in the C. crescentus cell cycle is regulated at the transcriptional level. The effect of altering the time of gene duplication in the cell cycle was examined by subcloning the complete hook operon on a plasmid that replicates throughout the S phase. The normal periodicity of flaK transcription and translation was maintained in this merodiploid strain, which suggests that replication alone is not sufficient to initiate flagellar gene expression. We also show that the three adjacent transcriptional units III, IV and V are required in trans for transcription of the book operon, and we discuss the possible role of these genes in the hierarchical regulation of the flagellar gene expression.

Social adjustment and interaction after severe head injury

The social adjustment of 11 severely head injured patients was assessed using the KATZ adjustment scale. The result suggested that the group had poor social adjustment, and on many dimensions was similar to a psychiatric population. The head-injured patients also had social interaction difficulties as assessed by a range of observational and self-report measures. When compared to an out-patient and non-clinical group they had poor social performance, high social anxiety and low self-esteem. The implications of this for rehabilitation are discussed.

Physical mapping and complementation analysis of transposon Tn5 mutations in Caulobacter crescentus: organization of transcriptional units in the hook gene cluster

Using the cloned DNA from the hook protein gene region of Caulobacter crescentus ( Ohta et al., Proc. Natl. Acad. Sci. U.S.A. 79:4863-4867, 1982), we have identified and physically mapped 19 Tn5-induced and 2 spontaneous insertion mutations to this region of the chromosome. These nonmotile mutants define a major cluster of fla genes that covers approximately 17 kilobases on the chromosome (hook gene cluster). Complementation analysis of the mutants using DNA fragments from the region subcloned in the broad host range plasmid pRK290 has shown that these fla genes are organized into at least five transcriptional units (I to V). Transcriptional unit II contains at least one gene in addition to the hook protein gene, which makes this the first operon described in C. crescentus. Expression of the hook protein gene and the genetically unlinked flagellin A and B genes by this set of mutants also furnishes additional insights into the hierarchial regulation of flagellar genes. We have found that the spontaneous insertion mutant ( SC511 ) of the hook protein gene ( flaK ) makes no flagellin A or B and that genes downstream from the hook protein gene are required in trans for expression of the hook protein operon and the flagellin A and B genes. Recombination and complementation results thus place flaK , flaJ , flaN , and flaO (R. C. Johnson and B. Ely , J. Bacteriol . 137:627-634, 1979) in the hook gene cluster, identify at least three new genes ( flbD , flbG , and flbF ), and suggest that this cluster may contain several additional, as yet unidentified, fla genes.

Isolation of flagellated membrane vesicles from Caulobacter crescentus cells: evidence for functional differentiation of polar membrane domains

An immunoaffinity chromatography procedure is described for the separation of membrane vesicles from Caulobacter crescentus cells into flagellated (polar) vesicles and nonflagellated (nonpolar) vesicles. Analysis by two-dimensional gel electrophoresis shows that a number of proteins are associated primarily with either the polar or the nonpolar fraction, and this result suggests that the envelope of these cells is organized into at least two relatively stable domains. Radioimmunoassay also shows that the membrane pool of flagellin, which is known to behave as a precursor in the assembly of the flagellar filament, may be localized exclusively in the polar membrane domain. Thus, the results provide biochemical evidence for the structural and functional differentiation of the C. crescentus cell envelope. These findings are consistent with a model we proposed previously to explain the targeting of surface structures to the new cell pole of C. crescentus. The immunoadsorption approach described here should be useful in the further investigation of this problem, as well as in the fractionation of membrane domains with characteristic surface antigens in other systems.

Evidence that subcellular flagellin pools in Caulobacter crescentus are precursors in flagellum assembly

To study the assembly of the Caulobacter crescentus flagellar filament, we have devised a fractionation protocol that separates the cellular flagellin into three compartments: soluble, membrane, and assembled. Radioactive labeling in pulse-chase and pulse-labeling experiments has demonstrated for the first time that both soluble and membrane-associated flagellin pools are precursors in the assembly of the flagellar filament. The results of these experiments also indicate that flagellar filament assembly occurs via the translocation of newly synthesized flagellins from the soluble pool to the membrane pool to the assembled flagellar filaments. It is not possible to conclude whether the soluble flagellin fraction is synthesized cytoplasmically or as a loosely associated membrane intermediate which is released during lysis. It is clear, however, that the soluble and membrane flagellins are in physically and functionally distinct pools. The implications of these findings for the study of protein secretion from cells and the invariant targeting of flagellar proteins to the stalk-distal pole of the dividing cell during flagellum morphogenesis are discussed.

Isolation and expression of cloned hook protein gene from Caulobacter crescentus

Previous genetic analysis of Caulobacter crescentus showed that the periodic synthesis of hook protein, flagellin A, and flagellin B, the major flagellar subunits, is coupled in some way to chromosome replication. To examine the regulation of flagellar gene expression at the molecular level, we isolated the gene that codes for the 72,000-dalton hook protein. A specific 125I-labeled anti-hook protein IgG was used to screen a hybrid lambdaL47.1 bank of 4,500 clones and to compare peptide maps of the cloned gene product with purified hook protein. Restriction analysis of DNA from the positive lambda clones and plasmid subclones showed that the structural gene for the hook protein is contained on a 2.3-kilobase (kb) BamHI fragment. The direction of transcription was established by demonstrating the inducibility of hook protein gene in strains with the 2.3-kb fragment fused to the Escherichia coli lipoprotein gene-lactose gene promoter-operator region of pIN-II. Preliminary genomic analysis showed that the hook gene occupies a single location on the C. crescentus chromosome. These results suggest that the periodic expression of the hook protein gene in the cell cycle does not involve a major or persistent rearrangement of the 2.3-kb coding sequence during the cell cycle.

Inhibition of herpes simplex virus multiplication by activated macrophages: a role for arginase?

Proteose-peptone-activated mouse macrophages can prevent productive infection by herpes simplex virus in neighboring cells in vitro whether or not those cells belong to the same animal species. The effect does not require contact between the macrophages and the infected cells, may be prevented by adding extra arginine to the medium, and may be reversed when extra arginine is added 24 h after the macrophages. Arginase activity was found both intracellularly and released from the macrophages. The extracellular enzyme is quite stable; 64% activity was found after 48 h of incubation at 37 degrees C in tissue culture medium. No evidence was found that the inefficiency of virus replication in macrophages was due to self-starvation by arginase. As might be predicted macrophages can, by the same mechanism, limit productive infection by vaccinia virus.

Circular organization of the DNA synthetic pathway in Caulobacter crescentus

Genetic analysis of the cell cycle of Caulobacter crescentus has identified a DNA synthetic pathway and a cell division pathway (M. A. Osley and A. Newton, J. Mol. Biol. 138:109-128, 1980). The results presented here show that in double-shift experiments the function of the PC2076 gene product, which is required for the initiation of DNA synthesis, depends on completion of a late stage of chromosome replication in the previous cell cycle. These findings suggest a circular organization of steps in the DNA synthetic pathway in C. crescentus.

Localization of surface structures during procaryotic differentiation: role of cell division in Caulobacter crescentus

Asymmetric cell division in Caulobacter crescentus produces two cell types, a stalked cell and a new swarmer cell, with characteristics surface structures. We have examined the role of the cell cycle in the differentiation of these two cells using adsorption of bacteriophage phi LC72, the assembly of the polar flagellum, and stalk formation as assays for changes in surface morphology. Previous studies of this aquatic bacterium [17,25] have suggested that the replicating chromosome acts as a "clock' in timing the formation of the flagellar filament at one pole of the new swarmer cell. the analysis of conditional cell cycle mutants presented here extends these results by showing that DNA synthesis is also required for adsorption of phage phi LC72 and, more importantly, they also suggest that a late cell division step is involved in determining the spatial pattern in which the phage receptors and flagella are assembled. We propose that this cell division step is required for formation of "organizational' centers which direct the assembly of surface structures at the new cell poles, and for the polarity reversal in assembly that accompanies swarmer cell to stalked cell development.

Localization of proteins in the inner and outer membranes of Caulobacter crescentus

Cytoplasmic and outer membranes of Caulobacter crescentus were separated by isopycnic sucrose gradient centrifugation into two peaks with buoyant densities 1.22 and 1.14 g/cm3. These peaks were identified as outer and cytoplasmic membranes by the enrichment of malate dehydrogenase and NADH oxidase in the lower density peak and the presence of flagellin, a cell surface protein, in the heavier peak. The identity of the heavier peak as outer membrane was confirmed by labeling of cells with diazotized [35S]sulfanilic acid, a reagent that does not penetrate intact cells. Under these conditions only outer membrane proteins were substituted by the sulfanilic acid. The distribution of proteins between the cytoplasmic and outer membranes were examined by the analysis of [35S]methionine-labeled membranes by SDS-polyacrylamide and two-dimensional gel electrophoresis. These results showed that the inner and outer membranes contain approximately equal numbers of proteins, and that the distribution of these proteins between the two layers is highly asymmetric. Although many of the proteins could be assigned to one or the other membrane fraction, a number of the outer membrane proteins in the 32 000-100 000 molecular weight range frequently contaminate the inner membrane fractions. The implications of these results for membrane isolation and separation in C. crescentus are discussed.

Purification and characterization of a polyhook protein from Caulobacter crescentus

A polyhook-producing strain of Caulobacter crescentus was isolated, and the polyhook protein was purified. The antigenicity and morphology of the polyhook structure are similar to the wild-type hook except that the mutant strain produces a hook structure at least 10-fold the length of wild-type hooks (1.0 versus 0.1 micrometers). The molecular weight of the polyhook protein, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, is 72,000, and the protein has a pI of approximately 6.1. Antibodies prepared against the polyhook protein were used to show that this protein is antigenically distinct from the Caulobacter flagellins. Amino acid analysis of the polyhook protein revealed compositional similarities to other gram-negative, bacterial hook proteins.

Regulation of cell cycle events in asymmetrically dividing cells: functions required for DNA initiation and chain elongation in Caulobacter crescentus

To study the regulation of cell cycle events after asymmetric cell division in Caulobacter crescentus, we have identified functions that are required for DNA synthesis in the stalked cell produced at division and in the new stalked cell that develops from the swarmer cell 60 min after division. The initiation of DNA synthesis in the two progeny cells is dependent upon at least two common functions. One of these is a requirement for protein synthesis and the other is a gene product identified in a temperature-sensitive cell cycle mutant. DNA chain elongation requires a third common function. The characteristic pattern of DNA synthesis in C. crescentus appears to be controlled in part by the expression of these functions in the two stalked cells at different times after cell division. The age distribution for Caulobacter cells in an exponential population has been calculated (Appendix by Robert Tax) and used to analyze some of the results.

Polyadenylic acid synthesis activity of purified DNA-dependent RNA polymerase from Caulobacter

Characterization of purified DNA-dependent RNA polymerase (EC 2.7.7.6) of Caulobacter crescentus, strain CB15 has led to the conclusion that this enzyme catalyzes poly(A) synthesis in the absence of template. Poly(A) synthetase activity co-purifies with both holoenzyme and core polymerase on DNA-cellulose columns, and core polymerase purified to 98% homogeneity by glycerol gradient centrifugation is still capable of catalyzing poly(A) polymerization. Both RNA synthesis and poly(A) polymerization activities are sensitive to rifampicin. In addition, RNA polymerase purified from partially rifampicin-sensitive mutants exhibits the same partial sensitivity in vitro to the drug in the synthesis of RNA and poly(A). The enzyme used in these studies was prepared by a simple method which allows a high yield of pure RNA polymerase from large batches of exponential cells. The procedure includes high speed centrifugation of cell extracts, DEAE-cellulose column, DNA-affinity chromatography, and low salt glycerol gradient centrifugation. Holoenzyme can be resolved into core and sigma subunit by either DNA-cellulose chromatography or glycerol gradient centrifugation, and the latter step allows recovery of pure sigma factor.

Characterization of unstable poly (A)-RNA in Caulobacter crescentus

A significant amount of poly(A)-RNA in Caulobacter crescentus is located on polysomes and the size distribution of this polysomal poly(A)-RNA is small compared to the total pulse-labeled RNA in these cells. These observations suggest that the poly (A)-RNA represents a subset of small messenger RNA molecules. Poly (A)-RNA, and presumably the poly (A) portion of these molecules is extremely unstable: as assayed by binding to oligo (dT)-cellulose the poly (A)-RNA turns over with a chemical half-life of 15--20 s compared to a half-life of approx. 2 min for total cellular messenger RNA. The presence of adenosine in hydrolysates of poly(A) tracts showed that these sequences are located at the 3'-OH end of the RNA. The ratios of AMP/adenosine in the samples confirmed that the length of the A-tracts is approx. 13-17 nucleotides (Ohta, N., Sanders, M. and Newton, A. (1975) Proc. Natl. Acad. Sci. U.S.A. 72, 2343--2346)

Reconstitution and purification of flagellar filaments from Caulobacter crescentus

Filaments from isolated flagella of Caulobacter crescentus have been purified by successive dissociation and reconstitution. After the second and third reconstitutions from subunits in 0.8 M sodium citrate, filament preparations contained only two proteins, flagellin A (26,000 daltons) and flagellin B (28,000 daltons). There was some enrichment for flagellin A during reconstitution by this procedure, since isolated flagella contained flagellin A and flagellin B in a ratio of approximately 3.8:1 and filaments after the third reconstitution contained the two proteins in a ratio of 5.0:1.

Regulation of flagellin synthesis in the cell cycle of caulobacter: dependence on DNA replication

Synthesis of the two filament proteins (flagellin A and flagellin B) of the Caulobacter creascentus flagellum was measured during the cell cycle. Synchronous cells were pulse-labeled with 36S-methionine, and flagellin proteins were isolated from crude extracts by radioimmune precipitation. The results showed that both proteins are maximally induced during the G2 period and that their induction requires de novo transcription. Flagellin A, however, continues to be made in the progeny swarmer cells after flagellin B synthesis has stopped. This discoordination in flagellin A and B synthesis and the relative abundance of the two proteins may result in part from the longer functional half-life of the flagellin A messenger RNA. Analysis of temperature-sensitive DNA chain elongation mutants suggests that the periodicity of flagellin A and B synthesis in the cell cycle is controlled by a late cell cycle event, presumably the completion of chromosome replication.

Mutational analysis of developmental control in Caulobacter crescentus

The relationship between the cell cycle and control of development has been studied by a genetic analysis of Caulobacter crescentus. The behavior of conditional cell division mutants showed that cell cycle events, such as DNA replication and cell division, are organized into a dependent pathway(s), i.e., later steps cannot preceed until earlier ones are completed. The ability of these strains to develop normally under nonpermissive conditions suggested that flagellin synthesis and stalk formation are dependent on the completion of different cell-cycle steps: the periodicity of flagellin synthesis is achieved by coupling it to either DNA chain elongation or completion, and stalk formation is dependent on a later step in the cell-division pathway. These developmental events are not themselves required for cell division, however.

Requirement of a cell division step for stalk formation in Caulobacter crescentus

Penicillin G at low concentrations blocked cell division in Caulobacter crescentus without inhibiting cell growth. The long filamentous cells formed after two to three generations under these conditions had a stalk at one pole and usually one or more flagella at the opposite pole. The failure of the filaments to form a second stalk at the flagellated pole indicates that stalk formation was dependent upon completion of a step that was also required for cell division. Two observations support this conclusion. (i) Penicillin did not stop the normal development of synchronous swarmer cells into stalked initiation and stalk elongation. (ii) When the action of penicillin was reversed by the addition of penicillinase to cultures of filaments, stalks were not formed at the nonstalked pole until after cell division had occurred; thus the normal order of development events was maintained: cell division leads to stalk formation. These results are consistent with a model in which the organization of the developmental program for stalk formation occurs before cell division as a consequence of steps that branch from the cell division pathway.

Poly(adenylic acid) sequences in the RNA of Caulobacter crescenus

Poly(adenylic acid) sequences have been isolated from the Gram-negative bacterium Caulobacter crescentus. Most of these A-rich tracts are associated with large RNA molecules that constitute an important fraction of the unstable RNA in these bacteria, and, as estimated by poly(U) filter binding, they are not present in the 16S or 23S ribosomal RNA. Preliminary estimates of size from polyacrylamide gel electrophoresis suggest that the majority of the A-rich tracts ranges from 15 to approximately 50 residues in length.

Gene transfer in Caulobacter crescentus: polarized inheritance of genetic markers

Recombination frequencies were determined for 15 independently isolated auxotrophs of C. crescentus crossed pairwise in all possible combinations. The results indicate that the mutants may be grouped into at least two types: "fertile" strains, which recombine with all other mutants at frequencies ranging from less than 10-6 to 3 times 10-2, and "nonfertile" strains which recombine with fertile strains at high frequencies and with other nonfertile strains at low or negligible frequencies. Several lines of evidence indicate a polarized inheritance of markers. Two of these are (1) the preferential inheritance of unselected markers from the nonfertile parent in fertile times nonfertile crosses, and (2) the consistent ordering of markers based on the frequency at which the mutants recombine with each of the three fertile strains. Although the evidence is not conclusive at this point, the results are most consistent with conjugation at the mechanism of gene transfer in these bacteria.

Dependence of cell division on the completion of chromosome replication in Caulobacter

The relationship between chromosome replication and cell division in the stalked bacterium Caulobacter crescentus has been investigated. Two compounds, hydroxyurea and mitomycin C, were found to inhibit completely deoxyribonucleic acid (DNA) synthesis while allowing continued cell growth and elongation. When these inhibitors were added to exponentially growing cultures, cell division stopped after 38 min when hydroxyurea was used and after 33 min when mitomycin C was used. The period of continued cell division corresponds closely to the period previously determined for the postsynthetic gap (G2) in the DNA cycle of this organism. These results indicate that cell division is coupled to the completion of chromosome replication in C. crescentus.

Role of transcription in the temporal control of development in Caulobacter crescentus (stalk-rifampin-RNA synthesis-DNA synthesis-motility)

The requirement for transcription during development of the stalked bacterium, Caulobacter crescentus, was studied with synchronous cultures of swarmer cells. The developmental pattern in these bacteria was first established by determination of the times at which specific changes in cell structure and function occurred. These changes could be divided into those characteristic of (a) development of the swarmer cell into the stalked cell: loss of motility and synthesis of the stalk, and (b) development of the stalked cell into the asymmetric dividing cell: chromosome replication, synthesis of the flagellum, motility, and division. The effect of rifampin in blocking several of these steps-loss of motility, initiation of chromosome replication, and cell division-indicates that RNA synthesis is required throughout the cell cycle for normal differentiation.

Operon coordination in different bacterial hosts

Coordination of gene expression in the lac operon was compared in Escherichia coli and Salmonella typhimurium as an approach to detecting possible differences in protein synthesis or membrane structure between organisms. Either a wild-type F' lac pro(AB) episome or the same episome with a polar mutation in one of the lac genes was introduced into pro(-) derivatives of the two strains of bacteria. Activity assays showed that the beta-galactosidase levels were only slightly lower in the S. typhimurium cells than in E. coli cells, whereas the transacetylase levels were significantly higher in S. typhimurium for all of the lac markers tested. Galactoside transport activities were always comparable in the two strains of bacteria; this latter result indicates that the cell envelopes of E. coli and S. typhimurium do not differ sufficiently to affect the membrane-associated lac transport system. It was found, however, that the specific transport activity is very sensitive to culture age in both bacteria, and decreases rapidly in cultures past the mid-exponential phase of growth.

Iron transport in Escherichia coli: relationship between chromium sensitivity and high iron requirement in mutants of Escherichia coli

Utilization of iron (Fe(3+)) by Escherichia coli depends upon a system which is determined by at least two genetic loci. Mutants which carry a deletion of the tonB-trp region of the chromosome grow only when very high concentrations of iron are present in the medium. These strains are sensitive to chromic ion (Cr(3+)) and, unlike the parent strain, fail to grow on MnSO(4) when FeSO(4) is not added to the medium. A second type of mutant, Chr2, which was isolated on the basis of its sensitivity to chromic ion, also requires a high concentration of iron for growth. This mutant can be distinguished phenotypically from the deletion mutants since it grows normally on low concentrations of iron, provided citrate is added to the medium. The chromium sensitivity of both types of mutants can be reversed by high concentrations of exogenous iron. The data are interpreted to indicate that the E. coli mutants studied are defective in iron transport and that residual iron transport is in some way inhibited by chromic ion.

Iron transport in Escherichia coli: roles of energy-dependent uptake and 2,3-dihydroxybenzoylserine

Escherichia coli strains B/r and 2276 contain an active transport system for iron. The system is energy-dependent, repressed by excess iron in the growth medium, and capable of accumulating iron inside of the cells at concentrations 2,000-fold higher than those in the medium. Two tonB-trp deletion mutants, strains B/rlt and B/lt7, which are sensitive to chromic ion and require high levels of iron for normal growth, are deficient in this active transport system. A point mutant, strain Chr2, which is also sensitive to chromic ion and requires high levels of iron for growth, has the active uptake system but cannot synthesize a specific chelator for iron, 2,3-dihydroxybenzoylserine (DHBS). Evidence is presented to support the hypothesis that both the active uptake system and chelation of iron by DHBS play a role in iron uptake from iron-deficient medium. The chromium sensitivity of the mutants can be explained by inhibition of uptake of exogenous iron.