Genetic relationship of two highly studied Synechococcus strains designated Anacystis nidulans

The cyanobacteria Synechococcus sp. strain PCC 7942 and Synechococcus sp. strain PCC 6301 are very closely related and both have been designated by the binomial Anacystis nidulans. The only established difference between the two strains is the superior transformation properties of strain PCC 7942. Significant homology between the rRNA genes of these strains was demonstrated by the ability of an rRNA operon from strain PCC 6301, interrupted by a spectinomycin and streptomycin resistance marker, to transform strain PCC 7942 by recombining with and replacing an endogenous rRNA operon. Restriction fragment length polymorphism data indicated that the chromosomes of the two strains were conserved around the three psbA loci, the two rRNA operons, and the psbDI locus. However, multiple polymorphisms were detected downstream of the psbDII locus, identifying a DNA rearrangement such as an inversion, insertion, or deletion within the chromosome. Analysis of genome structure by pulsed-field gel electrophoresis of large NotI restriction fragments showed only two bands that were visibly shifted between the chromosomes of the two strains. These data support their very close genetic relationship and the feasibility of studying genes derived from strain PCC 6301 in the highly transformable PCC 7942 strain.