Genomic Sequence and Transcriptional Analysis of a 23-Kilobase Mycobacterial Linear Plasmid: Evidence for Horizontal Transfer and Identification of Plasmid Maintenance Systems

Abstract

ABSTRACT Linear plasmids were unknown in mycobacteria until recently. Here, we report the complete nucleotide sequence of 23-kb linear plasmid pCLP from Mycobacterium celatum , an opportunistic pathogen. The sequence of pCLP revealed at least 19 putative open reading frames (ORFs). Expression of pCLP genes in exponential-phase cultures was determined by reverse transcriptase PCR (RT-PCR). Twelve ORFs were expressed, whereas no transcription of the 7 other ORFs of pCLP was detected. Five of the 12 transcribed ORFs detected by RT-PCR are of unknown function. Sequence analysis revealed similar loci in both M. celatum pCLP and the Mycobacterium tuberculosis chromosome, including transposase-related sequences. This result suggests horizontal transfer between these two organisms. pCLP also contains ORFs that are similar to genes of bacterial circular plasmids involved in partition ( par operon) and postsegregational ( pem operon) mechanisms. Functional analysis of these ORFs suggests that they probably carry out similar maintenance roles in pCLP.