(105e) A Bistable Switch Controls Drug Resistance Transfer in Enterococcus Faecalis Via Antisense RNA, Transcriptional Interference: An in Silico and In Vivo Approach

Recent analyses of prokaryotic and eukaryotic genomes have indicated the presence of large number of cis sense-antisense transcripts. Typically such sense ?antisense RNA pairs originate from overlapping genes with convergent face to face promoters, and are speculated to be involved in gene regulation. In this work, we present a novel mechanism of gene-regulation due to convergent transcription from the prgX/prgQ operon in Enterococcus faecalis controlling the initiation of conjugation-based transfer of antibiotic resistance plasmid pCF10 (encoding tetracycline resistance) from donor cells to recipient cells. Using mathematical modeling and experimentation, we show that convergent transcription in the prgX/prgQ operon results in a bistable genetic switch. We identify two novel mechanisms that attribute such a hysteretic behavior to the system: (i) Transcriptional Interference (TI), which results in pre-mature termination of elongating counter transcripts due to collision of elongating RNA polymerases transcribing from opposite directions, and (ii) Antisense Regulation (AR) between sense-antisense counter transcripts. We developed a discrete mathematical model to quantify the effect of RNA polymerase collision on transcription of the gene pair. We show, for the first time, that convergent transcription results in generation of truncated RNA of discrete sizes within the overlapping region both in the sense and antisense direction. In silico and in vivo data indicate that some of these small truncated RNA are rich in secondary structure and are involved in and Antisense Regulation (AR) due to interaction between counter transcripts. Incorporating sense-antisense interaction between full length sense transcripts and truncated antisense transcripts in the model; we demonstrate that both TI and AR are essential for a robust ?bistable switch-like? behavior as supported by our experimental data. Such a coupled effect of TI and AR reciprocally regulates expression from prgQ (induces conjugation) and prgX gene (represses conjugation), thus controlling the transfer of drug resistance in Enterococcus faecalis. Moreover, given that sense/antisense pairs are conserved across species, the coupled effect of TI and AR is likely to have a significant regulatory role in gene expression. Our model suggests that convergent transcription provides a novel mechanism of gene regulation attributing ?switch-like? characteristics.