The chromosome structure and the cell cycle of Caulobacter crescentus. Isolation and analysis of envelope-free nucleoids.

Envelope-free nucleoids were isolated from an asymmetrically dividing bacterium, Caulobacter crescentus. In the nucleoid fraction, most of the DNA and nascent RNA in the cell and about 2% of the total cellular proteins were recovered. The sedimentation coefficient of the nucleoid was constant (1260 S) during the G1 period of the swarmer cell cycle and increased to 1940 S during the S period. Since both replicating (S period) and non-replicating (G1 period) chromosomes had a similar superhelical concentration, the increase in the sedimentation coefficient was simply explained by duplication of the nucleoid structure. The duplicated nucleoid was shown to segregate prior to the cell division. The pulse-labeled proteins recovered in the nucleoid fraction contained several stage-specific species, most of which were detected at the beginning of S period.