Precise amounts of a novel member of a phosphotransferase superfamily are essential for growth and normal morphology in Caulobacter crescentus

The Caulobacter crescentus chromosomal clp locus contains the genes encodin g the components of ClpXP, a multisubunit protease required for cell cycle progression in this organism. Here, we report the identification and charac terization of cicA, a gene located between the clpX and clpP genes on the C aulobacter chromosome. cicA is a novel morphogene in C. crescentus and, lik e clpX and clpP, is essential for growth. A conditional cicA mutant stopped growth, but retained viability under restrictive conditions. In contrast, an increased concentration of CicA led to an immediate loss of the normal r od shape, an almost 10-fold increase of the cell's volume and a cell divisi on block. In parallel with this drastic morphological change, cells rapidly lost viability. Primary sequence analysis suggested that the cicA gene enc odes a member of a large superfamily of phosphotransferases, that include p hosphoserine phosphatases, the ATPase domain of P-type ATPases and receiver domains of response regulators. Four conserved motifs of this protein fami ly that have been implicated in the catalysis of phosphotransfer reactions were investigated by site-directed mutagenesis and were found to be critica l for in vivo function of CicA, Based on our observations, we postulate tha t CicA is involved in essential phosphotransferase reactions in Caulobacter and that increased activity of CicA has a deleterious effect on cell wall biosynthesis, morphogenesis and cell division.