DISTRIBUTION AND HOMOLOGY OF PLASMIDS IN SEVERAL STRAINS OF CYANOTHECE

The plasmid distribution of several clonal isolates of the unicellular , diazotrophic, cyanobacterium Cyanothece sp. has been analyzed. The C yanothece isolates contain three to four plasmids ranging in size from 4.8 kb to 40 kb. The plasmid profiles of three Cyanothece strains (BH 63, BH68, BH93) indicated that strains BH68 and BH93 were closely rela ted and that strain BH63 may be more distantly related. A small 4.8-kb plasmid (pSE480), from the clonal isolate Cyanothece sp. strain BH68F , has been subcloned and restriction mapped. Ten restriction sites hav e been mapped, five of which are unique and suitable for further subcl oning. Southern hybridization revealed that this plasmid was present i n two out of five clonal isolates of strain BH68 and in one isolate of strain BH93. A 10-kb plasmid from strain BH68F (pSE1000) was found in all of the BH68 isolates and was absent in the BH93 isolate, Cyanothe ce sp. strain BH93A. No notable physiological changes were observed in the absence of either the 4.8-kb or 10-kb plasmids. Therefore, these plasmids remain cryptic. Further analysis of these plasmids may provid e insight into the function of these plasmids and will allow the const ruction of shuttle vectors for gene transfer experiments.