SUMMARY: The complete nucleotide sequence of pSR1, a 3 kb multicopy cryptic plasmid from ATCC 19223 has been determined. pSR1 is unrelated to the 4.4 kb plasmid pBL1 and shows no DNA sequence conservation with plasmids from Transposon insertion and deletion mutants located the minimal replicon to within a 2.1 kb - restriction fragment. This region contains a single large open reading frame, ORF2, flanked at the 5 end by a series of inverted repeat sequences which may modulate its expression, and at the 3’ end by a region which may contain a replication origin. ORF2 (position 1633-2636) with a maximum coding potential of 36 kDa is essential for pSR1 replication and was designated the gene. The predicted ORF2 protein product exhibits 47% identity over a length of 343 amino acids with a replication-associated ORF in the plasmid pNG2, many of the changes being in the third base position. This observation suggests that pSR1 and pNG2, which are two plasmids from environmentally separated species, may share a common ancestral gene.


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