Isolation and characterization of two cryptic plasmids in the ammonia-oxidizing bacterium Nitrosomonas sp strain ENI-11

Two plasmids were discovered in the ammonia-oxidizing bacterium Nitrosomona s sp. strain ENI-11, which was isolated from activated sludge. The plasmids , designated pAYS and pAYL, were relatively small, being approximately 1.9 kb long. They were cryptic plasmids, having no detectable plasmid-linked an tibiotic resistance or heavy metal resistance markers. The complete nucleot ide sequences of pAYS and pAYL were determined, and their physical maps wer e constructed. There existed two major open reading frames, ORF1 in pAYS an d ORF2 in pAYL, each of which was more than 500 bp long. The predicted prod uct of ORF2 was 28% identical to part of the replication protein of a Bacil lus plasmid, pBAA1, However, no significant similarity to any known protein sequences was detected with the predicted product of ORF1, pAYS and pAYL h ad a highly homologous region, designated HHR, of 262 bp. The overall ident ity was 98% between the two nucleotide sequences. Interestingly, HHR homolo gous sequences,were also detected in the genomes of ENI-11 and the plasmidl ess strain Nitrasomonas europaea IFO14298, Deletion analysis of pAYS and pA YL indicated that HHR, together with either ORF1 or ORF2, was essential for plasmid maintenance in ENI-11. To our knowledge, pAYS and pAYL are the fir st plasmids found in the ammonia-oxidizing autotrophic bacteria.