The Caulobacter crescentus smc gene is required for cell cycle progressionand chromosome segregation

The highly conserved SMC (Structural Maintenance of Chromosomes) proteins f unction in chromosome condensation, segregation, and other aspects of chrom osome dynamics in both eukaryotes and prokaryotes. A nun mutation in the Ca ulobacter crescentus sme gene is conditionally lethal and causes a cell cyc le arrest at the predivisional cell stage. Chromosome segregation In wild-t ype and sme null mutant cells was examined by monitoring the intracellular localization of the replication origin and terminus by using fluorescence i n situ hybridization, In wild-type cells, the origin is located at the flag ellated pole of swarmer cells and, immediately after the initiation of DNA replication in stalked cells, one of the origins moves to the opposite pole , giving a bipolar localization of the origins. The terminus moves from the end of the swarmer cell opposite the origin to midcell. A subpopulation of the sme null mutant cells had mislocalized origins or termini, showing tha t the sme null mutation gives DNA segregation defects. Nucleoid morphology was also abnormal. Thus, we propose that the Caulobacter chromosomal origin s have specific cellular addresses and that the SMC protein plays important roles in maintaining chromosome structure and in partitioning. The specifi c cell cycle arrest in the sme null mutant Indicates the presence of a cell cycle checkpoint that senses perturbations in chromosome organization or s egregation.