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Background  Each  Caulobacter crescentus  cell division yields two distinct cell types: a flagellated swarmer cell and a non-motile stalked cell. The swarmer cell is further distinguished from the stalked cell by an inability to reinitiate DNA replication, by the physical properties of its nucleoid, and its discrete program of gene expression. Specifically, with regard to the latter feature, many of the genes involved in DNA replication are not transcribed in swarmer cells.    Results  We show that for one of these genes involved in DNA replication,  gyrB , its pattern of temporal expression depends upon an 80 base pair promoter region with strong resemblance to the  Caulobacter crescentus  σ 73  consensus promoter sequence; regulation does not appear to be affected by the general strength of the promoter activity, as mutations that increased its conformity with the consensus did not affect its cell-cycle expression pattern. Transcription from the  gyrB  promoter  in vitro  required only the presence of the σ 73  RNA polymerase (from  E. coli ) and the requisite nucleoside triphosphates, although a distinct binding activity, present in crude whole-cell extracts, formed a complex  gyrB  promoter DNA. We also assayed the effect on  gyrB  expression in strains containing mutations in either  smc  or  dps , two genes encoding proteins that condense DNA. However we found there was no change in the temporal pattern of  gyrB  transcription in strains containing deletions in either of these genes.    Conclusion  These experiments demonstrate that  gyrB  transcription does not require any auxiliary factors, suggesting that temporal regulation is not dependent upon an activator protein. Swarmer-specific silencing may not be attributable to the observed physical difference in the swarmer cell nucleoid, since mutations in either  smc  or  dps , two genes encoding proteins that condense DNA, did not alter the temporal pattern of  gyrB  transcription in strains containing deletions in either of these genes. Rather a repressor that specifically recognizes sequences in the  gyrB  promoter region that are also probably essential for transcription, is likely to be responsible for controlling cell cycle expression.

Role of core promoter sequences in the mechanism of swarmer cell-specific silencing of gyrB transcription in Caulobacter crescentus