Sites of positive and negative regulation in the Bacillus subtilis antiterminators LicT and SacY

The Bacillus subtilis homologous transcriptional antiterminators LicT and SacY control the inducible expression of genes involved in aryl beta-glucoside and sucrose utilization respectively. Their RNA-binding activity is carried by the N-terminal domain (CAT), and is regulated by two similar C-terminal domains (PRD1 and PRD2), which are the targets of phosphorylation reactions catalysed by the phosphoenolpyruvate: sugar phosphotransferase system (PTS). In the absence of the corresponding inducer, LicT is inactivated by BglP, the PTS permease (EII) specific for aryl beta-glucosides, and SacY by SacX, a negative regulator homologous to the EII specific for sucrose. LicT, but not SacY, is also subject to a positive control by the general PTS components EI and HPr, which are thought to phosphorylate LicT in the absence of carbon catabolite repression. Construction of SacY/LicT hybrids and mutational analysis enabled the location of the sites of this positive regulation at the two phosphorylatable His207 and His269 within LicT-PRD2, and suggested that the presence of negative charges at these sites is sufficient for LicT activation in vivo. The BglP-mediated inhibition process was found to essentially involve His100 of LicT-PRD1, with His159 of the same domain playing a minor role in this regulation. In vitro experiments indicated that His100 could be phosphorylated directly by the general PTS proteins, this phosphorylation being stimulated by phosphorylated BglP. We confirmed that, similarly, the corresponding conserved His99 residue in SacY is the major site of the negative control exerted by SacX on SacY activity. Thus, for both antiterminators, the EII-mediated inhibition process seems to rely primarily on the presence of a negative charge at the first conserved histidine of the PRD1.