Synthesis of specific membrane proteins is a function of DNA replication and phospholipid synthesis in Caulobacter crescentus

Analysis of the effects on membrane function and protein composition of altering phospholipid synthesis in Caulobacter crescentus showed that, like other bacteria, C. crescentus continues to induce a lactose transport system and to synthesize most membrane proteins. However, we show that the incorporation of a set of outer membrane proteins primarily synthesized in stalked cells is dependent on DNA replication which, in turn, is dependent on membrane phospholipid synthesis. Furthermore, the incorporation of another set of membrane proteins, two of which are synthesized primarily in the swarmer cell, appears to be independent of the replication of the chromosome but to be directly dependent on phospholipid synthesis. We have also found that when phospholipid synthesis is blocked, the synthesis of the flagellar proteins is inhibited and that this effect may be mediated by the primary inhibition of DNA replication. Newton has presented evidence that the synthesis of flagellar proteins is dependent on specific execution points in DNA replication and that this connection serves as a temporal regulator of differential protein synthesis (Osley et al., 1977; Sheffery & Newton, 1981). We suggest here that a direct link between the replicating chromosome and the growing membrane might serve, in turn, to dictate the site of membrane assembly of newly synthesized gene products.     

Regulatory interactions between phospholipid synthesis and DNA replication in Caulobacter crescentus.

Several Caulobacter crescentus mutants with lesions in phospholipid biosynthesis have DNA replication phenotypes. A C. crescentus mutant deficient in glycerol 3-phosphate dehydrogenase activity (gpsA) blocks phospholipid synthesis, ceases DNA replication, and loses viability in the absence of a glycerol phosphate supplement. To investigate the interaction between membrane synthesis and DNA replication during a single cell cycle, we moved the gpsA mutation into a synchronizable, but otherwise wild-type, strain. The first effect of withholding supplement was the cessation of synthesis of phosphatidylglycerol, a major component of the C. crescentus membrane. In the absence of glycerol 3-phosphate, DNA replication was initiated in the stalked cell at the correct time in the cell cycle and at the correct site on the chromosome. However, after replication proceeded bidirectionally for a short time, DNA synthesis dropped to a low level. The cell cycle blocked at a distinct middivision stalked cell, and this was followed by cell death. The "glycerol-less" death of the gpsA mutant could be prevented if the cells were treated with novobiocin to prevent the initiation of DNA replication. Our observations suggest that the processivity of C. crescentus replication requires concomitant phospholipid synthesis and that cell death results from incomplete replication of the chromosome.