The complete sequence and functional analysis of pANL, the large plasmid of the unicellular freshwater cyanobacterium Synechococcus elongatus PCC 7942

Two endogenous plasmids are present in Synechococcus elongatus PCC 7942, a model organism for studying photosynthesis and circadian rhythms in cyanobacteria. The large plasmid, pANL, was shown previously to be involved in adaptation of S. elongatus cells to sulfur starvation, which provided the first evidence of cellular function of a cyanobacterial plasmid. Here, we report the complete sequence of pANL, which is 46,366 bp in length with 53% GC content and encodes 58 putative ORFs. The pANL plasmid can be divided into four structural and functional regions: the replication origin region, a signal transduction region, a plasmid maintenance region, and a sulfur-regulated region. Cosmid-based deletion analysis suggested that the plasmid maintenance and replication origin regions are required for persistence of pANL in the cells. Transposon-mediated mutagenesis and complementation-based pANL segregation assays confirmed that two predicted toxin–antitoxin cassettes encoded in the plasmid maintenance region, belonging to PemK and VapC families, respectively, are necessary for plasmid exclusion. The compact and efficient organization of sulfur-related genes on pANL may provide selective advantages in environments with limited sulfur.