LacR Is a Repressor oflacABCDand LacT Is an Activator oflacTFEG, Constituting thelacGene Cluster in Streptococcus pneumoniae

Abstract

ABSTRACTComparison of the transcriptome ofStreptococcus pneumoniaestrain D39 grown in the presence of either lactose or galactose with that of the strain grown in the presence of glucose revealed the elevated expression of various genes and operons, including thelacgene cluster, which is organized into two operons, i.e.,lacoperon I (lacABCD) andlacoperon II (lacTFEG). Deletion of the DeoR family transcriptional regulatorlacRthat is present downstream of thelacgene cluster revealed elevated expression oflacoperon I even in the absence of lactose. This suggests a function of LacR as a transcriptional repressor oflacoperon I, which encodes enzymes involved in the phosphorylated tagatose pathway in the absence of lactose or galactose. Deletion oflacRdid not affect the expression oflacoperon II, which encodes a lactose-specific phosphotransferase. This finding was further confirmed by β-galactosidase assays with PlacA-lacZand PlacT-lacZin the presence of either lactose or glucose as the sole carbon source in the medium. This suggests the involvement of another transcriptional regulator in the regulation oflacoperon II, which is the BglG-family transcriptional antiterminator LacT. We demonstrate the role of LacT as a transcriptional activator oflacoperon II in the presence of lactose and CcpA-independent regulation of thelacgene cluster inS. pneumoniae.