Caulobacter crescentus is a gram-negative, oliogtrophic bacterium. It is an important model organism for studying the regulation of the cell cycle, asymmetric cell division, and cellular differentiation.
The complete genome sequence of C. crescentus was determined by the whole genome random sequencing method. The genome consists of a circular chromosome of 4,016,942 base pairs encoding 3,767 genes.
The control of cell cycle progression has been shown to depend on the differential availability and activation by phosphorylation of the two-component system response regulator CtrA and the CckA histidine kinase. Both of these regulators control the time of chromosome replication initiation, DNA methylation mediated by the CcrM DNA methyltransferase, cell division, and flagella and pili biogenesis. Critical to cell progression is the proteolysis of CtrA∼P at the G1-S transition.
Bacteria regulate proteolysis of specific proteins to control cell cycle progression and morphogenesis. For both the response regulator CtrA and the chemoreceptor McpA, residues at or near the C terminus are necessary for proper cell-dependent turnover. Nearly 20% of all response regulators, and more than 50% of all cell cycle regulated response regulators and histidine kinases, have C-terminal residues of AA, IA, or VA.
Two-Component Signal Transduction Proteins
C. crescentus has the largest number of signal transduction proteins of any sequenced bacterium when adjusted for genome size. Analysis of the genome sequence revealed:
- 34 histidine protein kinase (HPK) genes
- 44 response regulator (RR) genes
- 27 hybrid HPK/RR genes
A number of cell cycle-regulated HPKs and RRs are essential for cell viability. Eleven of the hybrid HPK/RR proteins are predicted to be cytoplasmic, as are 13 of the nonhybrid HPKs. Many of these cytoplasmic kinases contain PAS domains, which are often involved in sensing changes in cellular energy levels, oxygen levels, or redox potential. It appears that C. crescentus relies heavily on molecular networks that sense and respond to intracellular oxygen and redox state.
Chromosome methylation of the N-6 adenine of the sequence GAnTC is catalyzed by the CcrM DNA methyltransferase. The transcription of the ccrM gene is under tight cell cycle control; the CcrM protein is present only in the predivisional cell, when it is available to bring the two newly replicated chromosomes from the hemi- to the full methylation state. CcrM is essential for viability, and its expression at inappropriate times in the cell cycle causes defects in cell division and DNA replication. Temporarily regulated methylation of GAnTC site is a component of cell cycle progression. There are only 4,496 of these sites in the genome and 22% are located between the ORFs that comprise 90.6% of the genome, which supports the argument that methylation of these sites plays a key regulatory role.
There are 16 putative RNA polymerase sigma factors in the C. crescentus genome. Two of these extracytoplasmic function (ECF) sigma factors, SigT and SigU, are specifically transcribed at the swarmer-to-stalked cell transition and are components of the genetic network that controls cell cycle progression. Nineteen percent of C. crescentus genes’ transcription have been shown to be cell cycle regulated. The RNA polymerase holoenzyme containing RpoN (sigma 54) is used for the transcription of genes involved in cell differentiation.