Abstract
The new small cryptic plasmid pNMO propagating in Lactobacillus casei IAM 1045 was isolated from a naturally occurring Lactobacillus strain N1f. Sequencing analysis revealed that the pNMO genome is a double-stranded DNA of 3522bp, and has two possible open reading frames, rep and pre as well as three other minor ORFs. The predicted rep product (Rep) of 318 amino acids bears close resemblance to other presumed replication proteins (Rep) encoded by the Lactobacillus plasmids such as pLP1, replicating by the rolling-circle (RC) mechanism via single-stranded (SS) DNA intermediates. Upstream rep, pNMO carries two putative replication origins: one is a Rep-mediated nick-site for the plus-strand synthesis (+ori), and the other is a single strand origin (sso) for the minus-strand replication. Preliminary in vivo experiments suggested that pNMO generates SS DNA intermediates in the N1f cells. On the other hand, the putative pNMO pre product (Pre) of 361 amino acids shows extensive similarity to other presumed Pre/Mov (plasmid recombination/ mobilization) proteins encoded by the Lactobacillus plasmids pLB4 and pLAB1000. These results suggested that pNMO replicates via the RC mechanism, and carries the Pre/Mov system.
