Molecular genetics of Rhodobacter sphaeroides 2.4.1: Genome structure, plasmid characterization and chromosome transfer

Abstract

Physical mapping techniques were employed to construct both a physical and genetic maps of the Rhodobacter sphaeroides 2.4.1 genome. It was found that this $\alpha$-purple, nonsulfur photosynthetic bacterium carries two different circular chromosomes whose sizes are 3046 $\pm$ 95 and 914 $\pm$ 17 kb in addition to five endogenous plasmids with a total size of approximately 450 kb.

The smallest, 42 kb, endogenous plasmid was observed to be self transmissible. As little as an 425 bp oriT-containing DNA fragment isolated from this plasmid was able to confer transfer, between R. sphaeroides strains, when it was cloned into an otherwise non-transmissible plasmid. The DNA sequence of this oriT containing fragment reveals an A-T rich region, several direct and inverted repeats, as well as putative IHF-binding sites.

Random insertion of oriT into the R. sphaeroides 2.4.1 genome generated Hfr-like strains which exhibit a gradient of marker transfer, and has further provided classical genetic evidence for the circularity as well as the presence of two distinct chromosomal linkage groups in this bacterium.