Convergent and overlapping transcripts of the Chlamydia trachomatis 7.5-kb plasmid

Transcription of the 7.5-kb cryptic plasmid of Chlamydia trachomatis serovar L2 was investigated. Faint, diffuse transcripts of about 1.6 and 2.2 kb and intense short transcripts of about 250 and 430 bases were identified by Northern blot analysis. The short transcripts were found to have a common 5' end corresponding to bp 501 relative to the unique BamHI site of the plasmid and to terminate at different downstream sites. Putative promoter sequences of TTGCCA and TATATT, which closely resemble the consensus recognition site of Escherichia coli sigma 70, were identified at the -35 and -10 positions upstream from the 5' end of the short transcripts made in chlamydia. Transcripts of similar sizes were also expressed from this promoter in E. coli harboring a recombinant plasmid encoding the short transcripts. The short transcripts encode a common open reading frame (ORF) of 34 codons; however, a strong ribosome binding site was not found in the vicinity of the initiator codon, and it is not known whether the transcripts are translated in vivo. A large ORF of 330 codons, which has been shown to encode a hypothetical protein containing conserved domains of recombinase-like proteins, is antisense to the short transcripts. Transcripts encoding the large ORF could not be detected directly by Northern blot or primer extension analysis. However, transcripts were detected by polymerase chain reaction amplification of the large ORF cDNA and when Southern blots of single-stranded antisense DNA for the large ORF were probed with radiolabeled RNA synthesized by host-free chlamydial reticulate bodies. Thus, both strands of the chlamydial plasmid are transcribed in the region encoding the short transcripts. We propose that the short transcripts play a regulatory role as antisense RNAs.