REQUIREMENT OF TOPOISOMERASE-IV PARC AND PARE GENES FOR CELL-CYCLE PROGRESSION AND DEVELOPMENTAL REGULATION IN CAULOBACTER-CRESCENTUS

We have identified the parC and parE genes encoding DNA topoisomerase IV (Topo IV) in Caulobacter crescentus. We have also characterized the effect of conditional Topo IV mutations on cell division and morpholo gy. Topo IV mutants of C. crescentus are unlike mutants of Escherichia coli and S. typhimurium, which form long filamentous cells that are d efective in nucleoid segregation and divide frequently to produce anuc leate cells. Topo IV mutants of C. crescentus are highly pinched at mu ltiple sites (cell separation phenotype) and they do not divide to pro duce cells lacking DNA. These results suggest unique regulatory mechan isms coupling nucleoid partitioning and cell division in this aquatic bacterium. In addition, distinctive nucleoid-partitioning defects are not apparent in C. crescentus Topo IV mutants as they are in E. coli a nd S. typhimurium. However, abnormal nucleoid segregation in parE muta nt cells could be demonstrated in a genetic background containing a co nditional mutation in the C. crescentus ftsA gene, an early cell divis ion gene that is epistatic to parE for cell division and growth. We di scuss these results in connection with the possible roles of C. cresce ntus Topo IV in the regulation of cell division, chromosome partitioni ng, and late events in polar morphogenesis. Although the ParC and ParE subunits of Topo IV are very similar in sequence to the GyrA and GyrB subunits of DNA gyrase, we have used DNA sequence analysis to identif y a highly conserved 'GyrA box' sequence that is unique to the GyrA pr oteins and may serve as a hallmark of the GyrA protein family.