Sequence analysis and characterizations of two novel plasmids isolated from Thermus sp. 4C

Two novel plasmids, named pS4C and pL4C, were isolated from the thermophilic bacterium Thermus sp. 4C. The pS4C with a length of 5015 bp and 58.25% of G+C content, contains 9 putative open reading frames (ORFs). The larger plasmid, pL4C, consisting of 21,248 bp, has a G+C content of 68.60% and 34 putative ORFs. Both plasmids encode their own replication protein. The ORF 22 of pL4C and the ORF 4 of pS4C encode proteins with high sequence similarities to integrase (97%) and transposase (97%), respectively, which are both involved in DNA rearrangement and exchange. Furthermore, sequence analysis of pL4C also showed that several plasmid-encoded genes may be involved in DNA modification and repair, such as DNA G:T-mismatch repair endonuclease and micrococcal nuclease-like protein. These proteins may be involved in raising the repair efficiency and other minor editing needs. Interestingly, the elimination of plasmids significantly lowered the growth temperature of Thermus sp. 4C. Few reports dealing with the DNA repair enzymes on the plasmid from Thermus strains were published so far.