THE DNA-SEQUENCE AND MINIMAL REPLICON OF THE CORYNEBACTERIUM-GLUTAMICUM PLASMID PSR1 - EVIDENCE OF A COMMON ANCESTRY WITH PLASMIDS FROM C-DIPHTHERIAE

The complete nucleotide sequence of pSR1, a 3 kb multicopy cryptic pla smid from Corynebacterium glutamicum ATCC 19223 has been determined. p SR1 is unrelated to the 4.4 kb Brevibacterium lactofermentum plasmid p BL1 and shows no DNA sequence conservation with plasmids from Staphylo coccus. Transposon insertion and deletion mutants located the minimal replicon to within a 2.1 kb NcoI-BclI restriction fragment. This regio n contains a single large open reading frame, ORF2, flanked at the 5' end by a series of inverted repeat sequences which may modulate its ex pression, and at the 3' end by a region which may contain a replicatio n origin. ORF2 (position 1633-2636) with a maximum coding potential of 36 kDa is essential for pSR1 replication and was designated the rep g ene. The predicted ORF2 protein product exhibits 47 % identity over a length of 343 amino acids with a replication-associated ORF in the C. diphtheriae 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 Corynebacterium species, may s hare a common ancestral rep gene.