Dna Replication in Caulobacter Crescentus.

Abstract

Cell division in the Gram-negative bacterium Caulobacter crescentus is asymmetric, yielding two very different daughter cells. One daughter initiates chromosome replication immediately after cell division, the other delays initiation of chromosome replication for one-third of a generation after cell division. This unique regulation of DNA replication motivated a study of plasmid replicon activity in C. crescentus. The broad host range plasmid RP4 is maintained by C. crescentus as covalently closed circles at a minimum copy number of 1.7 per chromosome. Plasmids dependent on the origins of replication of the ColE1 plasmid, the pBR322 plasmid, or the Enterobacter aerogenes chromosome are not maintained by C. crescentus. Furthermore, the non-functional origins of replication, when integrated into RP4, have no detectable effect on the maintenance of such chimeric plasmids in C. crescentus. It had been suggested that DNA replication in C. crescentus is dependent on continuous phospholipid synthesis. The data presented here fail to confirm this suggestion. In fact when phospholipid synthesis was inhibited with the antibiotic cerulenin DNA replication was essentially normal. Furthermore, DNA was at its maximal rate at a time during the cell cycle when no net synthesis of phospholipids was apparent. These studies also uncovered an unexpected periodicity of phospholipid synthesis during the C. crescentus cell cycle. Phospholipid synthesis stops during the first half of chromosome replication and during cell division. In preparation for isolation and characterization of the C. crescentus chromosomal origin of replication a recombination deficient strain of C. crescentus was isolated. This strain was found among a collection of mutants sensitive to ultraviolet irradiation and was shown to carry out homologous recombination 5000-fold less frequently than in the wild type strain.