Osmoregulation in Lactococcus lactis: BusR, a transcriptional repressor of the glycine betaine uptake system BusA

Summary The busA ( opuA ) locus of Lactococcus lactis encodes a glycine betaine uptake system. Transcription of busA is osmotically inducible and its induction after an osmotic stress is reduced in the presence of gly- cine betaine. Using a genetic screen in CLG802, an Escherichia coli strain carrying a lacZ transcriptional fusion expressed under the control of the busA promoter, we isolated a genomic fragment from the L. lactis subsp. cremoris strain MG1363, which represses transcription from busA p . The cloned locus responsible for this repression was identified as a gene present upstream from the busA operon, encod- ing a putative DNA binding protein. This gene was named busR . Electrophoretic mobility shift and foot- printing experiments showed that BusR is able to bind a site that overlaps the busA promoter. Overex- pression of busR in L. lactis reduced expression of busA . Its disruption led to increased and essentially constitutive transcription of busA at low osmolarity. Therefore, BusR is a major actor of the osmotic reg- ulation of busA in L. lactis .