Extraordinary long-stem confers resistance of intrinsic terminators to processive antitermination

Many prokaryotic operons encode a processive antitermination (P-AT) system. Transcription complexes associated with an antitermination factor can bypass multiple transcription termination signals regardless of their sequences. However, to avoid compromising transcriptional regulation of downstream regions, the terminator at the end of the operon needs to be resistant to antitermination. So far, no studies on the mechanism of resistance to antitermination have been reported. The recently discovered conAn P-AT system is composed of two components that are encoded at the start of many conjugation operons on plasmids of Gram-positive bacteria. Here we report the identification of a conAn-resistant terminator, named Ter(R), in the conjugation operon of the Bacillus subtilis plasmid pLS20, re-defining the end of the conjugation operon. We investigated the various characteristics of Ter(R) and show that its extraordinary long stem is the determining feature for resistance to antitermination. This is the first P-AT resistance mechanism to be reported.