THE MARINE BACTERIUM PSEUDOALTEROMONAS-HALOPLANKTIS HAS A COMPLEX GENOME STRUCTURE COMPOSED OF 2 SEPARATE GENETIC UNITS

The genome size of Pseudoalteromonas haloplonktis, a ubiquitous and ea sily cultured marine bacterium, was measured as a step toward estimati ng the genome complexity of marine bacterioplankton. To determine tota l genome size, we digested P. haloplonktis DNA with the restriction en donucleases NotI and SfiI, separated the Fragments using pulsed-field gel electrophoresis (PFGE), and summed the sizes of the Fragments. The P. haloplonktis genome was 3512 +/- 112 kb by NotI digestion and 3468 +/- 54.1 kb by SfiI digestion. P. haloplanktis is also shown to have a complex genome structure, composed of two large replicons of similar to 2700 and 800 kb. Three pieces of evidence support this conclusion: (1) Two separate bands are always seen in PFGE of undigested P. halop lanktis DNA; (2) restriction digests of the larger band are mining a b and of similar to 650 kb compared with restriction digests of total ge nomic DNA; and (3) a 16S rDNA probe hybridized to the larger replicon but not to the smaller. To our knowledge, haloplanktis is the first ma rine bacterium shown to have a complex genome structure.