MalR-mediated regulation of the Streptococcus pneumoniae malMP operon at promoter PM -: Influence of a proximal divergent promoter region and competition between MalR and RNA polymerase proteins

The Streptococcus pneumoniae mal regulon contains two operons, malXCD and malMP involved in the uptake and utilization of maltosaccharides. Both operons are transcribed from two divergent promoters, P-X and P-M, and are negatively regulated by the MalR transcriptional repressor. Purified MalR protein binds to two DNA regions that encompasses both promoters, thus occupying its two operators, O-M and O-X. However, the levels of occupation and repression were different, being higher when MalR was bound to O-M than when it was anchored to O-X. Competition experiments between MalR and the Escherichia coli RNA polymerase on promoters P-M and P-X showed that the affinity of either protein for the promoter/operator DNA sequences was important to determine the frequency of transcription initiation. In addition to the control exerted by MalR, expression from promoter P-M was affected by upstream sequences located within or close to P-X promoter.