Temporal control of the cell cycle in Caulobacter crescentus: roles of DNA chain elongation and completion

A NAD-dependent glutamate dehydrogenase coordinates metabolism with cell division in Caulobacter crescentus

Coupling cell cycle with nutrient availability is a crucial process for all living cells. But how bacteria control cell division according to metabolic supplies remains poorly understood. Here, we describe a molecular mechanism that coordinates central metabolism with cell division in the α-proteobacterium Caulobacter crescentus. This mechanism involves the NAD-dependent glutamate dehydrogenase GdhZ and the oxidoreductase-like KidO. While enzymatically active GdhZ directly interferes with FtsZ polymerization by stimulating its GTPase activity, KidO bound to NADH destabilizes lateral interactions between FtsZ protofilaments. Both GdhZ and KidO share the same regulatory network to concomitantly stimulate the rapid disassembly of the Z-ring, necessary for the subsequent release of progeny cells. Thus, this mechanism illustrates how proteins initially dedicated to metabolism coordinate cell cycle progression with nutrient availability.

A temperature-sensitive mutation in the dnaE gene of Caulobacter crescentus that prevents initiation of DNA replication but not ongoing elongation of DNA

A genetic screen for cell division cycle mutants of Caulobacter crescentus identified a temperature-sensitive DNA replication mutant. Genetic complementation experiments revealed a mutation within the dnaE gene, encoding the alpha-catalytic subunit of DNA polymerase III holoenzyme. Sequencing of the temperature-sensitive dnaE allele indicated a single base pair substitution resulting in a change from valine to glutamic acid within the C-terminal portion of the protein. This mutation lies in a region of the DnaE protein shown in Escherichia coli, to be important in interactions with other essential DNA replication proteins. Using DNA replication assays and fluorescence flow cytometry, we show that the observed block in DNA synthesis in the Caulobacter dnaE mutant strain occurs at the initiation stage of replication and that there is also a partial block of DNA elongation.

Cell-cycle-regulated expression and subcellular localization of the Caulobacter crescentus SMC chromosome structural protein

Structural maintenance of chromosomes proteins (SMCs) bind to DNA and function to ensure proper chromosome organization in both eukaryotes and bacteria. Caulobacter crescentus possesses a single SMC homolog that plays a role in organizing and segregating daughter chromosomes. Approximately 1,500 to 2,000 SMC molecules are present per cell during active growth, corresponding to one SMC complex per 6,000 to 8,000 bp of chromosomal DNA. Although transcription from the smc promoter is induced during early S phase, a cell cycle transcription pattern previously observed with multiple DNA replication and repair genes, the SMC protein is present throughout the entire cell cycle. Examination of the intracellular location of SMC showed that in swarmer cells, which do not replicate DNA, the protein forms two or three foci. Stalked cells, which are actively engaged in DNA replication, have three or four SMC foci per cell. The SMC foci appear randomly distributed in the cell. Many predivisional cells have bright polar SMC foci, which are lost upon cell division. Thus, chromosome compaction likely involves dynamic aggregates of SMC bound to DNA. The aggregation pattern changes as a function of the cell cycle both during and upon completion of chromosome replication.

Dynamic localization of proteins and DNA during a bacterial cell cycle

A cellular differentiation programme that culminates in an asymmetric cell division is an integral part of the cell cycle in the bacterium Caulobacter crescentus. Recent work has uncovered mechanisms that ensure the execution of many events at different times during the cell cycle and at specific places in the cell. Surprisingly, in this one-micron bacterial cell, the dynamic spatial disposition of regulatory proteins, structural proteins and specific regions of the chromosome are important components of both cell-cycle progression and the generation of daughter cells with different cell fates.

New members of the ctrA regulon: the major chemotaxis operon in Caulobacter is CtrA dependent

The Caulobacter crescentus che promoter region consists of two divergent promoters, directing expression of the major chemotaxis operon and a novel gene cagA (chemotaxis associated gene A). Analyses of start sites by primer extension and alignment of the divergent promoters revealed significant similarities between them at the -35 promoter region. Both mcpA and cagA are differentially expressed in the cell cycle, with maximal activation of transcription in predivisional cells. The main difference between the mcpA and cagA promoters is that, in common with the fljK flagellin, cagA is expressed in swarmer cells. A cagA--lacZ promoter fusion that contains 36 bases of untranslated mRNA has sufficient information to segregate the lacZ transcript to swarmer cells. Expression of mcpA and cagA was dependent on DNA replication. Transcriptional epistasis experiments were performed to identify potential regulators in the flagellar hierarchy. The sigma factor RpoN, which is required for flagellar biogenesis, is not required for mcpA and cagA expression. Mutations in the genes for the MS-ring and the switch complex (flagellar class II mutants) do not affect expression of mcpA and cagA. However, CtrA, an essential response regulator of flagellar gene transcription, is required.

CtrA mediates a DNA replication checkpoint that prevents cell division in Caulobacter crescentus

Coordination of DNA replication and cell division is essential in order to ensure that progeny cells inherit a full copy of the genome. Caulobacter crescentus divides asymmetrically to produce a non-replicating swarmer cell and a replicating stalked cell. The global response regulator CtrA coordinates DNA replication and cell division by repressing replication initiation and transcription of the early cell division gene ftsZ in swarmer cells. We show that CtrA also mediates a DNA replication checkpoint of cell division by regulating the late cell division genes ftsQ and ftsA. CtrA activates transcription of the P(QA) promoter that co-transcribes ftsQA, thus regulating the ordered expression of early and late cell division proteins. Cells inhibited for DNA replication are unable to complete cell division. We show that CtrA is not synthesized in pre-divisional cells in which replication has been inhibited, preventing the transcription of P(QA) and cell division. Replication inhibition prevents the activation of the ctrA P2 promoter, which normally depends on CtrA phosphorylation. This suggests the possibility that CtrA phosphorylation may be affected by replication inhibition.

Identification, characterization, and chromosomal organization of cell division cycle genes in Caulobacter crescentus

We report a detailed characterization of cell division cycle (cdc) genes in the differentiating gram-negative bacterium Caulobacter crescentus. A large set of temperature-sensitive cdc mutations was isolated after treatment with the chemical mutagen N-methyl-N'-nitro-N-nitrosoguanidine. Analysis of independently isolated mutants at the nonpermissive temperature identified a variety of well-defined terminal phenotypes, including long filamentous cells blocked at various stages of the cell division cycle and two unusual classes of mutants with defects in both cell growth and division. The latter strains are uniformly arrested as either short bagel-shaped coils or large predivisional cells. The polar morphology of these cdc mutants supports the hypothesis that normal cell cycle progression is directly responsible for developmental regulation in C. crescentus. Genetic and physical mapping of the conditional cdc mutations and the previously characterized dna and div mutations identified at least 21 genes that are required for normal cell cycle progression. Although most of these genes are widely scattered, the genetically linked divA, divB, and divE genes were shown by genetic complementation and physical mapping to be organized in one gene cluster at 3200 units on the chromosome. DNA sequence analysis and marker rescue experiments demonstrated that divE is the C. crescentus ftsA homolog and that the ftsZ gene maps immediately adjacent to ftsA. On the basis of these results, we suggest that the C. crescentus divA-divB-divE(ftsA)-ftsZ gene cluster corresponds to the 2-min fts gene cluster of Escherichia coli.

Identification of a novel response regulator required for the swarmer-to-stalked-cell transition in Caulobacter crescentus

The onset of motility late in the Caulobacter crescentus cell cycle depends on a signal transduction pathway mediated by the histidine kinase PleC and response regulator DivK. We now show that pleD, whose function is required for the subsequent loss of motility and stalk formation by the motile swarmer cell, encodes a 454-residue protein with tandem N-terminal response regulator domains D1 and D2 and a novel C-terminal GGDEF domain. The identification of pleD301, a semidominant suppressor of the pleC Mot phenotype, as a mutation predicted to result in a D-53-->G change in the D1 domain supports a role for phosphorylation in the PleD regulator. Disruptions constructed in the pleD open reading frame demonstrated that the gene is not essential and that the pleC phenotype can also be suppressed by a recessive, loss-of-function mutation. These results suggest that PleD is part of a signal transduction pathway controlling stalked-cell differentiation early in the C. crescentus cell cycle.

Coordinate cell cycle control of a Caulobacter DNA methyltransferase and the flagellar genetic hierarchy

The expression of the Caulobacter ccrM gene and the activity of its product, the M.Ccr II DNA methyltransferase, are limited to a discrete portion of the cell cycle (G. Zweiger, G. Marczynski, and L. Shapiro, J. Mol. Biol. 235:472-485, 1994). Temporal control of DNA methylation has been shown to be critical for normal development in the dimorphic Caulobacter life cycle. To understand the mechanism by which ccrM expression is regulated during the cell cycle, we have identified and characterized the ccrM promoter region. We have found that it belongs to an unusual promoter family used by several Caulobacter class II flagellar genes. The expression of these class II genes initiates assembly of the flagellum just prior to activation of the ccrM promoter in the predivisional cell. Mutational analysis of two M.Ccr II methylation sites located 3' to the ccrM promoter suggests that methylation might influence the temporally controlled inactivation of ccrM transcription. An additional parallel between the ccrM and class II flagellar promoters is that their transcription responds to a cell cycle DNA replication checkpoint. We propose that a common regulatory system coordinates the expression of functionally diverse genes during the Caulobacter cell cycle.

Regulation of cellular differentiation in Caulobacter crescentus

In Caulobacter crescentus, asymmetry is generated in the predivisional cell, resulting in the formation of two distinct cell types upon cell division: a motile swarmer cell and a sessile stalked cell. These progeny cell types differ in their relative programs of gene expression and DNA replication. In progeny swarmer cells, DNA replication is silenced for a defined period, but stalked cells reinitiate chromosomal DNA replication immediately following cell division. The establishment of these differential programs of DNA replication may be due to the polar localization of DNA replication proteins, differences in chromosome higher-order structure, or pole-specific transcription. The best-understood aspect of Caulobacter development is biogenesis of the polar flagellum. The genes encoding the flagellum are expressed under cell cycle control predominantly in the predivisional cell type. Transcription of flagellar genes is regulated by a trans-acting hierarchy that responds to both flagellar assembly and cell cycle cues. As the flagellar genes are expressed, their products are targeted to the swarmer pole of the predivisional cell, where assembly occurs. Specific protein targeting and compartmentalized transcription are two mechanisms that contribute to the positioning of flagellar gene products at the swarmer pole of the predivisional cell.

Cell cycle arrest of a Caulobacter crescentus secA mutant

Cell differentiation is an inherent component of the Caulobacter crescentus cell cycle. The transition of a swarmer cell, with a single polar flagellum, into a sessile stalked cell includes several morphogenetic events. These include the release of the flagellum and pili, the proteolysis of chemotaxis proteins, the biogenesis of the polar stalk, and the initiation of DNA replication. We have isolated a group of temperature-sensitive mutants that are unable to complete this process at the restrictive temperature. We show here that one of these strains has a mutation in a homolog of the Escherichia coli secA gene, whose product is involved in protein translocation at the cell membrane. This C. crescentus secA mutant has allowed the identification of morphogenetic events in the swarmer-to-stalked cell transition that require SecA-dependent protein translocation. Upon shift to the nonpermissive temperature, the mutant secA swarmer cell is able to release the polar flagellum, degrade chemoreceptors, and initiate DNA replication, but it is unable to form a stalk, complete DNA replication, or carry out cell division. At the nonpermissive temperature, the cell cycle blocks prior to the de novo synthesis of flagella and chemotaxis proteins that normally occurs in the predivisional cell. Although interactions between the chromosome and the cytoplasmic membrane are believed to be a functional component of the temporal regulation of DNA replication, the ability of this secA mutant to initiate replication at the nonpermissive temperature suggests that SecA-dependent events are not involved in this process. However, both cell division and stalk formation, which is analogous to a polar division event, require SecA function.

An unusual promoter controls cell-cycle regulation and dependence on DNA replication of the Caulobacter fliLM early flagellar operon

Transcription of flagellar genes in Caulobacter crecentus is programmed to occur during the predivisional stage of the cell cycle. The mechanism of activation of Class II flagellar genes, the highest identified genes in the Caulobacter flagellar hierarchy, is unknown. As a step toward understanding this process, we have defined cis-acting sequences necessary for expression of a Class II flagellar operon, fliLM. Deletion analysis indicated that a 55 bp DNA fragment was sufficient for normal, temporally regulated promoter activity. Transcription from this promoter-containing fragment was severely reduced when chromosomal DNA replication was inhibited. Extensive mutational analysis of the promoter region from -42 to -5 identified functionally important nucleotides at -36 and -35, between -29 and -22, and at -12, which correlates well with sequences conserved between fliLM and the analogous regions of two other Class II flagellar operons. The promoter sequence does not resemble that recognized by any known bacterial sigma factor. Models for regulation of Caulobacter early flagellar promoters are discussed in which RNA polymerase containing a novel sigma subunit interacts with an activation factor bound to the central region of the promoter.

Expression of an early gene in the flagellar regulatory hierarchy is sensitive to an interruption in DNA replication

Genes involved in the biogenesis of the flagellum in Caulobacter crescentus are expressed in a temporal order and are controlled by a trans-acting regulatory hierarchy. Strains with mutations in one of these genes, flaS, cannot transcribe flagellar structural genes and divide abnormally. This gene was cloned, and it was found that its transcription is initiated early in the cell cycle. Subclones that restored motility to FlaS mutants also restored normal cell division. Although transcription of flaS was not dependent on any other known gene in the flagellar hierarchy, it was autoregulated and subject to mild negative control by other genes at the same level of the hierarchy. An additional level of control was revealed when it was found that an interruption of DNA replication caused the inhibition of flaS transcription. The flaS transcript initiation site was identified, and an apparently unique promoter sequence was found to be highly conserved among the genes at the same level of the hierarchy. The flagellar genes with this conserved 5' region all initiate transcription early in the cell cycle and are all sensitive to a disruption in DNA replication. Mutations in these genes also cause an aberrant cell division phenotype. Therefore, flagellar genes at or near the top of the hierarchy may be controlled, in part, by a unique transcription factor and may be responsive to the same DNA replication cues that mediate other cell cycle events, such as cell division.

The cell cycle-regulated flagellar gene flbF of Caulobacter crescentus is homologous to a virulence locus (lcrD) of Yersinia pestis

We have characterized flbF, a key locus located at the top of the flagellar gene hierarchy of Caulobacter crescentus. This gene is required for transcription from sigma 54 promoters of fla genes expressed late in the cell cycle. We have determined the nucleotide sequence of the gene, mapped the 5' end of the flbF RNA, and examined the pattern of expression in the cell cycle. Our results show that flbF is expressed earlier in the cell cycle than other fla genes, that it is expressed at a low level throughout the stalked cell cycle, and that its 5' regulatory region contains sequences that can be aligned with the sigma 28 promoter consensus reported for enteric bacteria. flbF contains an open reading frame of 700 residues with an amino-terminal half rich in hydrophobic residues that could correspond to six to eight transmembrane domains. The translated flbF sequence is very similar to LcrD (low calcium response) encoded by virulence plasmids of pathogenic Yersinia spp. (G. Plano, S. Barve, and S. Straley, J. Bacteriol. 173:7293-7303, 1991). LcrD and FlbF can be aligned over the entire length of the proteins with the greatest degree of sequence identity (45%) in the hydrophobic amino-terminal region. The high degree of sequence homology of proteins derived from widely differing organisms, including Caulobacter and Yersinia species, suggests that FlbF and LcrD may be representatives of a larger family of regulatory proteins with a common sensor mechanism for modifying responses to appropriate stimuli.

Cloning and cell cycle-dependent expression of DNA replication gene dnaC from Caulobacter crescentus

Chromosome replication in the asymmetrically dividing bacteria Caulobacter crescentus is discontinuous with the new, motile swarmer cell undergoing an obligatory presynthetic gap period (G1 period) of 60 min before the initiation of DNA synthesis and stalk formation. To examine the regulation of the cell division cycle at the molecular level, we have cloned the DNA chain elongation gene dnaC from a genomic DNA library constructed in cosmid vector pLAFR1-7. To ensure that the cloned sequence corresponded to dnaC, we isolated the gene by genetic complementation of the temperature-sensitive allele dnaC303 on DNA fragment that contained a Tn5 insertion element tightly linked by transduction to dnaC. The size of the dnaC gene was estimated to be 1,500 bp or less based on the pattern of complementation by subcloned restriction and BAL 31 deletion fragments. Nuclease S1 assays were used to map the transcription start site and to determine the pattern of dnaC expression in the cell cycle. Large amounts of the dnaC transcript began to accumulate only in the late G1 period of the swarmer cell and then peaked early during chromosome replication. We confirmed that the gene is periodically transcribed by monitoring the rate of beta-galactosidase synthesis directed by a dnaC promoter-lacZ fusion in a synchronous cell culture. dnaC is the first C. crescentus cell cycle gene whose regulation has been reported, and the discontinuous pattern of its expression suggests that the DNA synthetic period in these dimorphic bacteria is regulated in part by the stage-specific expression of DNA replication genes.

Negative transcriptional regulation in the Caulobacter flagellar hierarchy

The Caulobacter crescentus flagellum is formed at a specific time in the cell cycle and its assembly requires the ordered expression of a large number of genes. These genes are controlled in a positive trans-acting hierarchy that reflects the order of assembly of the flagellum. Using plasmids carrying transcriptional fusions of either a neo or a lux reporter gene to the promoters of three flagellar genes representing different ranks in the hierarchy (the hook operon, a basal body gene flbN, and the flaO gene), we have measured the level of chimeric gene expression in 13 flagellar mutant backgrounds. Mutants in the hook operon or in basal body genes caused overproduction of both hook operon and basal body gene chimeric mRNAs, suggesting that negative regulation is superimposed on the positive trans-acting control for these early events in the flagellar hierarchy. Mutants in the structural genes and in genes involved in flagellar assembly had no effect on flaO expression, placing the flaO gene near the top of the hierarchy. However, flaO expression appears to be under negative control by two regulatory genes flaS and flaW. Negative control, as a response to the completion of specific steps in the assembly process, may be an important mechanism used by the cell to turn off flagellar gene expression once the gene product is no longer needed.

Cell-cycle-dependent polar morphogenesis in Caulobacter crescentus: roles of phospholipid, DNA, and protein syntheses

During swarmer cell differentiation in Caulobacter crescentus, morphogenesis at the swarmer pole is characterized by the loss of the flagellum, by the loss of phage receptor activity (PRA) (the ability of the cell to adsorb phage phi CbK), and finally by the initiation of stalk outgrowth at the site formerly occupied by the flagellum and the PRA. We show here that each of these events is a cell cycle-dependent event requiring continuous protein synthesis for its execution but occurring normally in the absence of DNA synthesis or phospholipid synthesis. During stalked-cell differentiation, the flagellum and PRA reappear and the stalk elongates considerably. We show here that these events are also cell cycle dependent, requiring not only de novo protein synthesis but also DNA and phospholipid syntheses. When synchronous cells dividing 160 min after collection were used, PRA reappearance occurred at 110 min. This PRA reappearance was dependent on a phospholipid synthesis-requiring event occurring at 70 min, a DNA synthesis-requiring event occurring at 95 min, and a protein synthesis-requiring event occurring at 108 min. In the absence of net phospholipid synthesis, stalk elongation appeared more or less normal, but the stalks eventually became fragile, and by 240 min, most of the stalks had broken off, leaving only stubs attached to the cell body.

Characterization of strains containing mutations in the contiguous flaF, flbT, or flbA-flaG transcription unit and identification of a novel fla phenotype in Caulobacter crescentus

During the Caulobacter crescentus cell cycle, flagellin synthesis and filament assembly are temporally controlled events which require the products encoded by the contiguous flaF, flbT, and flbA-flaG transcription units (P.V. Schoenlein, L.S. Gallman, and B. Ely, J. Bacteriol. 171:000-000, 1989). To better define the functions of these genes, immunoprecipitation studies, Western blot (immunoblot) analyses, and electron microscopic analyses characterized flagellin synthesis and assembly in mutant and merodiploid strains. Mutations in the flaF or flbA-flaG transcription unit resulted in reduced synthesis of the 25- and 27-kilodalton (kDa) flagellins. In contrast, mutations in flbT resulted in overproduction of these flagellins. The FlbT phenotype is unique, since all other identified C. crescentus fla mutations cause a reduction in the levels of the 25- and 27-kDa flagellins. Furthermore, the flbT mutant showed a chemotaxis deficiency even though it was motile. Thus, the flbT gene product appears to be involved in the regulation of both flagellin synthesis and chemotactic function. Mutations in the flbT and flbA-flaG transcription units also resulted in the production of a 22-kDa flagellin species that is not normally detected in wild-type cells. This flagellin species was not detected in the flbT filaments. Furthermore, the 22-kDa flagellin was no longer detected in flbA pseudorevertants that assembled functional filaments. Thus, the 22-kDa flagellin does not appear to be assembled into filaments. Since many of the flbT filaments are shorter than wild-type filaments, we discuss the possibility that the 22-kDa flagellin species may adversely affect flagellin assembly in this mutant.

Turning off flagellum rotation requires the pleiotropic gene pleD: pleA, pleC, and pleD define two morphogenic pathways in Caulobacter crescentus

We have identified mutations in three pleiotropic genes, pleA, pleC, and pleD, that are required for differentiation in Caulobacter crescentus. pleA and pleC mutants were isolated in an extensive screen for strains defective in both motility and adsorption of polar bacteriophage phi CbK; using temperature-sensitive alleles, we determined the time at which the two genes act. pleA was required for a short period at 0.7 of the swarmer cell cycle for flagellum biosynthesis, whereas pleC was required during an overlapping period from 0.6 to 0.95 of the cell cycle to activate flagellum rotation as well as to enable loss of the flagellum and stalk formation by swarmer cells after division. The third pleiotropic gene, pleD, is described here for the first time. A pleD mutation was identified as a bypass suppressor of a temperature-sensitive pleC allele. Strains containing this mutation were highly motile, did not shed the flagellum or form stalks, and retained motility throughout the cell cycle. Since pleD was required to turn off motility and was a bypass suppressor of pleC, we conclude that it acts after the pleA and pleC gene functions in the cell cycle. No mutants defective in both flagellum biosynthesis and stalk formation were identified. Consequently, we propose that the steps required for formation of swarmer cells and subsequent development into stalked cells are organized into at least two developmental pathways: a pleA-dependent sequence of events, responsible for flagellum biosynthesis in predivisional cells, and a pleC-pleD-dependent sequence, responsible for flagellum activation in predivisional cells and loss of motility and stalk formation in progeny swarmer cells.

Organization and temporal expression of a flagellar basal body gene in Caulobacter crescentus

Caulobacter crescentus assembles a single polar flagellum at a defined time in the cell cycle. The protein components of the flagellar hook and filament are synthesized just prior to their assembly. We demonstrated that the expression of a gene, flaD, that is involved in the formation of the flagellar basal body is under temporal control and is transcribed relatively early in the cell cycle, before the hook and flagellin genes are transcribed. Thus, the order of flagellar gene transcription reflects the order of assembly of the protein components. A mutation in the flaD gene results in the assembly of a partial basal body which is missing the outermost P and L rings as well as the external hook and filament (K.M. Hahnenberger and L. Shapiro, J. Mol. Biol. 194:91-103, 1987). The flaD gene was cloned and characterized by nucleotide sequencing and S1 nuclease protection assays. In contrast to the protein components of the hook and filament, the protein encoded by the flaD gene contains a hydrophobic leader peptide. The predicted amino acid sequence of the leader peptide of flaD is very similar to the leader peptide of the flagellar basal body P ring of Salmonella typhimurium (M. Homma, Y. Komeda, T. Iino, and R.M. Macnab, J. Bacteriol. 169:1493-1498, 1987).

Identification of two new cell division genes that affect a high-molecular-weight penicillin-binding protein in Caulobacter crescentus

Penicillin-binding proteins (PBPs) are membrane proteins associated with the synthesis of the bacterial cell wall. We report the characterization of 14 PBPs in Caulobacter crescentus, using in vivo and in vitro penicillin-binding assays and experiments to determine their possible role in cell division. New conditional cell cycle mutants were isolated by selecting cephalosporin-C-resistant mutants of the beta-lactamase strain SC1107 at 30 degrees C that are also defective in cell division at 37 degrees C. They fall into two classes, represented by strains PC8002 and PC8003. Strain PC8002 produced short cells arrested at all stages of cell division at 37 degrees C and was found to contain a high-molecular-weight PBP 1B which was temperature sensitive when assayed in vivo and in vitro. Strain PC8003 was blocked at an early stage of cell division and formed tightly coiled, unpinched filaments. This cephalosporin-C-resistant strain was also defective in PBP 1B, but only when assayed in vivo. PBP 1B behaved like a high-affinity PBP, and in competition assays, beta-lactams that induced filamentation bound preferentially to PBP 1B. These results and the phenotype of mutant PC8002 suggest that PBP 1B is required for cell division, as well as for cell growth, in C. crescentus. The behavior of strain PC8003 suggests that it contains a conditionally defective gene product that interacts in some way with PBP 1B at an early stage of cell division. None of the mutants showed an allele-specific PBP pattern when assayed in vitro at the nonpermissive temperature, but all of them displayed temperature-sensitive PBP 1C (102 kilodaltons) activity. Thus, it appears that PBP 1C is inhibited at 37 degree C as a consequence of filamentous growth.

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.

Identification of a gene cluster involved in flagellar basal body biogenesis in Caulobacter crescentus

The bacterial flagellum is a complex structure composed of a transmembrane basal body, a hook, and a filament. In Caulobacter crescentus the biosynthesis and assembly of this structure is under temporal and spatial control. To help to define the order of assembly of the flagellar components and to identify the genes involved in the early steps of basal body construction, mutants defective in basal body formation have been analyzed. Mutants in the flaD flaB flaC gene cluster were found to be unable to assemble a complete basal body. The flaD BC motC region was cloned and the genes were localized by subcloning and complementation analysis. A series of Tn5 insertion mutations in the flaD BC region were mapped. Complementation analysis of the Tn5 insertion mutants indicated the existence of at least four transcriptional units in the region and identified the presence of two new genes designated flbN and flbO. Mutants in flbN, flaB, flaC and flbO were unable to assemble any basal body structure and are likely to be involved in the early steps of basal body formation. The flaD mutant, however, was found to contain a partially assembled basal body consisting of the rod and three hook-distal rings. All of the mutants in this cluster exhibited pleiotropic effects on the expression of other flagellar and chemotaxis functions, including the level of synthesis of flagellins, the hook protein and hook protein precursor, and the level of chemotaxis methylation.

Growth of bacteria with dimorphic vegetative cell cycles

A mathematical model of the growth of bacteria with dimorphic cell cycles is described. In these bacteria motile swarmer cells differentiate to stalked reproductive cells and the proportions of these cell types change in a characteristic fashion during growth. The selection of parameters to fit the model to experimental data can result in the elucidation of the factors controlling the differentiation of swarmer cells.

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.

Caulobacter crescentus fatty acid-dependent cell cycle mutant

A fatty acid auxotroph of Caulobacter crescentus, AE6001, which displays a strict requirement for unsaturated fatty acids to grow on glucose as the carbon source has been isolated. Starvation of AE6001 for unsaturated fatty acids resulted in a block in the cell cycle. Starved cultures accumulated at the predivisional cell stage after a round of DNA replication had been completed and after a flagellum had been assembled at the pole of the cell. Cell division and cell growth failed to occur probably because the mutant was unable to synthesize a membrane. An analysis of double mutants containing the fatB503 allele and other mutations in membrane biogenesis demonstrated that the cell cycle of AE6001 blocked at a homeostatic state. The addition of oleic acid to starved cultures permitted cell division and the initiation of a new round of DNA replication. The coincident block in both the initiation of DNA replication and membrane assembly, exhibited by starved cultures of this mutant, suggests that the fatB503 gene product may be involved in the coordination of these events.

The association between prolyl hydroxylase metabolism and cell growth in cultured L-929 fibroblasts

Prolyl 4-hydroxylase (EC 1.14.11.2) is a key enzyme in collagen biosynthesis, its active form is a tetramer (alpha 2 beta 2). In L-929 fibroblasts in the log phase of culture there is a low level of active enzyme. When the cell culture reaches confluency, prolyl hydroxylase activity in cells increases by a process that requires de novo RNA and protein synthesis. The same result may be achieved by crowding the cells (replating log phase cells at the density of stationary phase cells). In the work reported here we further examined induction of the enzyme. RNA synthesis necessary for enzyme induction is complete 6 h after "crowding" while protein synthesis requires 12 h. Thymidine (0.2-0.5 mM) added to log phase cells will also cause enzyme induction to the level found in "crowded" or resting cells. We also looked at the decay of the enzyme activity after subculture. This occurs rapidly (enzyme half-life is 1-2 h) and is concurrent with the re-entry of resting cells into cell cycle; however, thymidine added at the time of subculture to block DNA synthesis does not prevent the loss of prolyl hydroxylase activity. These results suggest that when cells are not engaged in propagation, they begin to synthesize luxury proteins such as prolyl hydroxylase. However, the loss of prolyl hydroxylase during subculture is probably not a direct consequence of DNA synthesis.

Synthesis and assembly of flagellar components by Caulobacter crescentus motility mutants

Cultures of wild-type Caulobacter crescentus and strains with fla mutations representing 24 genes were pulse-labeled with 14C-amino acids and analyzed by immunoprecipitation to study the synthesis of flagellar components. Most fla mutants synthesize flagellin proteins at a reduced rate, suggesting the existence of some mechanism to prevent the accumulation of unpolymerized flagellin subunits. Two strains contain deletions that appear to remove a region necessary for this regulation. The hook protein does not seem to be subject to this type of regulation and, in addition, appears to be synthesized as a faster-sedimenting precursor. Mutations in a number of genes result in the appearance of degradation products of either the flagellin or the hook proteins. Mutations in flaA, -X, -Y, or -Z result in the production of filaments (stubs) that contain altered ratios of the flagellin proteins. In some flaA mutants, other flagellin-related proteins were assembled into the stub structures in addition to the flagellins normally present. Taken together, these analyses have begun to provide insight into the roles of individual fla genes in flagellum biogenesis in C. crescentus.

Hexamethylenebisacetamide-resistant murine erythroleukemia cells have altered patterns of inducer-mediated chromatin changes

We determined inducer-mediated changes in chromatin structure near the globin genes in a variant line of murine erythroleukemia cells (MELC). The variant cell line, R1, was derived from the inducer-sensitive DS19 cell line by selection for inducer-resistance. R1 cells are resistant to induction of erythroid differentiation by hexamethylenebisacetamide (HMBA) whereas the parental line is HMBA-sensitive. Uninduced MELC (both inducer-sensitive DS19 cells and inducer-resistant R1 cells) have DNase I-sensitive sites in chromatin containing the alpha 1- and beta maj-globin genes. These nuclease-sensitive regions are located within the beta maj-globin second intervening sequence (IVS2) and near the alpha 1-globin gene 5' cap site. Culture with HMBA causes changes in chromatin structure in both parental and variant cell lines. In DS19 cells, the DNase I-sensitive site within the beta maj-globin IVS2 becomes more resistant to nuclease cleavage, and a new DNase I-sensitive region develops near the beta maj-globin cap site. In addition, the nuclease-sensitive region adjacent to the cap site of the alpha 1-globin gene increases, and a novel 5' nuclease-sensitive site is also established. In R1 cells, HMBA-mediated changes in chromatin structure are incomplete. The DNase I-sensitive site within the beta maj-globin IVS2 becomes more resistant to nuclease cleavage, but the nuclease sensitivity near the beta maj-globin cap site does not increase to the extent observed in DS19 cells. The pattern of nuclease sensitivity near the alpha 1-globin gene is essentially unchanged after culture of R1 cells with HMBA. Thus, in R1 cells, resistance to HMBA-induced expression of globin genes is associated with failure to detect inducer-mediated changes in chromatin structure 5' to the cap site of the alpha 1- and beta maj-globin genes. These results also suggest that the increased nuclease resistance of a site in the beta maj-globin IVS2 does not depend on the establishment of a DNase I-sensitive region near the beta maj-globin gene cap site.

Developmental regulation of a sporulation-specific enzyme activity in Saccharomyces cerevisiae

An alpha-glucosidase activity (SAG) occurs in a/alpha Saccharomyces cerevisiae cells beginning at about 8 to 10 h after the initiation of sporulation. This enzyme is responsible for the rapid degradation of intracellular glycogen which follows the completion of meiosis in these cells. SAG differs from similar activities present in vegetative cells and appears to be a sporulation-specific enzyme. Cells arrested at various stages in sporulation (DNA replication, recombination, meiosis I, and meiosis II) were examined for SAG activity; the results show that SAG appearance depends on DNA synthesis and some recombination events but not on the meiotic divisions.

Cell surface patterning and morphogenesis: biogenesis of a periodic surface array during Caulobacter development

Shape changes, extended processes, and other surface elaborations are associated with cellular differentiation, and the cell membranes involved with these developmental changes often are reshaped without a major alteration in biochemical composition. Caulobacter crescentus produces a hexagonally-packed periodic surface layer that covers the entire cell and further, mimics some of the membrane-mediated changes of higher organisms by forming a membranous stalk during its distinctive life cycle. Growth of the surface layer was examined during the cell cycle by treating synchronously growing cells with surface layer antibody, continuing growth, and then labeling for electron microscopy with a protein A-colloidal gold conjugate. Three regions of distinctive surface array biogenesis were resolved. The periodic surface layer on the main cell body was enlarged by insertion of new material at numerous uniformly distributed points. In contrast, the surface layer on the stalk appeared as entirely new synthesis. In examining growth of the stalk in subsequent generations, we noted that growth of stalk surface persisted at the stalk-cell body junction. The region of cell division also showed a pattern of entirely new surface layer production at late stages in division, similar to the stalk. The immunocytological method also facilitated a careful examination of stalk initiation and growth. Although initiation was under precise temporal and spatial regulation, the rate of stalk elongation was variable from cell to cell and apparently no longer under cell cycle control. The similarity of surface layer biogenesis on the stalk and the site of cell division may be a significant reflection of other events occurring at the cell pole. A model suggested by this and other studies that can account for the temporal pattern of polar morphogenesis is discussed, as is the potential relationship between the geometrically ordered surface array and the formation or maintenance of the stalk.

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.

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.

Developmental regulation of a sporulation-specific enzyme activity in Saccharomyces cerevisiae

An alpha-glucosidase activity (SAG) occurs in a/alpha Saccharomyces cerevisiae cells beginning at about 8 to 10 h after the initiation of sporulation. This enzyme is responsible for the rapid degradation of intracellular glycogen which follows the completion of meiosis in these cells. SAG differs from similar activities present in vegetative cells and appears to be a sporulation-specific enzyme. Cells arrested at various stages in sporulation (DNA replication, recombination, meiosis I, and meiosis II) were examined for SAG activity; the results show that SAG appearance depends on DNA synthesis and some recombination events but not on the meiotic divisions.

The caulobacters: ubiquitous unusual bacteria

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