LmrR-mediated gene regulation of multidrug resistance in Lactococcus lactis

Multidrug resistance (MDR) in Lactococcus lactis is due to the expression of the membrane ATP-binding cassette (ABC) transporter LmrCD. In the absence of drugs, the transcriptional regulator LmrR prevents expression of the ImrCD operon by binding to its operator site. Through an autoregulatory mechanism LmrR also suppresses its own expression. Although the ImrR and ImrCD genes have their own promoters, primer extension analysis showed the presence of a long transcript spanning the entire ImrR ImrCD cluster, in addition to various shorter transcripts harbouring the ImrCD genes only. 'In-gel' Cu-phenanthroline footprinting analysis indicated an extensive interaction between LmrR and the ImrR promoter/operator region. Atomic force microscopy imaging of the binding of LmrR to the control region of ImrR DNA showed severe deformations indicative of DNA wrapping and looping, while LmrR binding to a fragment containing the ImrCD control region induced DNA bending. The results further suggest a drug-dependent regulation mechanism in which the ImrCD genes are co-transcribed with ImrR as a polycistronic messenger. This leads to an LmrR-mediated regulation of ImrCD expression that is exerted from two different locations and by distinct regulatory mechanisms.