Regulation of stalk elongation by phosphate in Caulobacter crescentus

In Caulobacter crescentus, stalk biosynthesis is regulated by cell cycle cu es and by extracellular phosphate concentration. Phosphate-starved cells un dergo dramatic stalk elongation to produce stalks as much as 30 times as lo ng as those of cells growing in phosphate-rich medium. To identify genes in volved in the control of stalk elongation, transposon mutants were isolated that exhibited a long-stalk phenotype irrespective of extracellular phosph ate concentration. The disrupted genes were identified as homologues of the high-affinity phosphate transport genes pstSCAB of Escherichia coli, In E. coli, pst mutants have a constitutively expressed phosphate (Pho) regulon. To determine if stalk elongation is regulated by the Pho regulon, the Caul obacter phoB gene that encodes the transcriptional activator of the Pho reg ulon was cloned and mutated. While phoB was not required for stalk synthesi s or for the cell cycle timing of stalk synthesis initiation, it was requir ed for stalk elongation in response to phosphate starvation, Both pstS and phoB mutants were deficient in phosphate transport. When a phoB mutant was grown with limiting phosphate concentrations, stalks only increased in leng th by an average of I. l-fold compared to the average 9-fold increase in st alk length of wild-type cells grown in the same medium. Thus, the phenotype s of phoB and pst mutants were opposite, phoB mutants were unable to elonga te stalks during phosphate starvation, whereas pst mutants made long stalks in both high- and low-phosphate media. Analysis of double pst phoB mutants indicated that the long-stalk phenotype of pst mutants was dependent on ph oB, In addition, analysis of a pstS-lacZ transcriptional fusion showed that pstS transcription is dependent on phoB, These results suggest that the si gnal transduction pathway that stimulates stalk elongation in response to p hosphate starvation is mediated by the Pst proteins and the response regula tor PhoB.