Characterization of the Streptococcus pneumoniae maltosaccharide regulator MalR, a member of the LacI-GalR family of repressors displaying distinctive genetic features

The gene encoding a transcriptional repressor of the maltosaccharide utilization operons of the Gram-positive bacterium Streptococcus pneumoniae (malR) has been cloned and sequenced. The genetic structure of the locus reveals the presence of an upstream gene necessary for growth on maltotetraose medium (malA). The phenotype of malR- and malA- mutants obtained by interruption of the coding regions suggests that both genes could belong to the same transcription unit. Two copies of a DNA motif consisting of three conserved regions of 59, 42, and 49 nucleotides were found located upstream and downstream of the malA-malR putative operon. These DNA structures are almost identical to the reported Box sequences associated with several genes of S. pneumoniae. The protein encoded by malR was visualized and partially purified after selective expression in Escherichia coli, whereas the product of malA was identified in vitro. The amino acid sequence of MalR displays similarities with the Lac and Gal family of repressors. The highest similarities were found when comparing MalR with the E. coli MalI repressor, which is related with an indirect induction pathway of the maltose regulon. The significance of these similarities is discussed in terms of the possible evolutionary pathways followed by structural and regulatory genes of sugar utilization systems in bacteria.