The bacterium Caulobacter crescentus coordinates the synthesis of polar structures with the progression of the cell division cycle. Much of this coordination is exerted by control of the global response regulator CtrA by a complex signal transduction pathway. We investigated the role of the signal transduction protein DivJ in polar development and cell division by analyzing spontaneous suppressor mutants. By finding mutants that compensate for the slow growth rate of a divJ mutant, we were able to find genes that encode proteins involved in the same pathway as DivJ, with regards to growth rate and cell division.

Upon analysis of our suppressor mutants, we demonstrated that DivJ normally plays a role in reducing the amount of CtrA∼P in C. crescentus cells, and that mutations in two other signal transduction proteins, DivL and CckA, result in a compensatory reduction of CtrA∼P. Our finding that DivL is involved in modulating the amount of CtrA∼P in C. crescentus confirmed earlier proposals that DivL was involved in the pathway culminating in CtrA phosphorylation. We also found that mutations in the signal transduction protein PleC suppress the slow growth and morphological defects of a divJ mutant. This result directly contradicts previously published results. We therefore constructed a double divJ, pleC deletion mutant and confirmed that a pleC deletion suppresses a divJ deletion. This result is in agreement with other data that describe DivJ and PleC function, and as such, greatly improves the strength of the working model of the large signal transduction pathway in C. crescentus.

The remainder of our work examined the nature of the suppressor mutant DivL proteins. Our results indicate that DivL is difficult to purify in soluble form, and that the three suppressor mutations further decrease the solubility of the proteins. We examined the ability of the mutants and wild-type DivL to autophosphorylate and found that the DivL22 mutant has a lower level of autophosphorylation than wild-type DivL or the DivL33 and DivL35 mutants. This suggests that DivL22 may reduce the amount of CtrA∼P in the cell by being less able to autophosphorylate and transfer the phosphoryl group to CtrA. However, DivL33 and DivL35 do not have a substantial autophosphorylation deficiency and must be acting to reduce cellular CtrA∼P levels in another way, perhaps by having an increase phosphatase activity towards CtrA∼P or by affecting DivK activity.