Like HPr, Crh becomes (de)phosphorylated in vitro at residue Ser46 by the metabolite-controlled HPr kinase/phosphorylase HPrK/P. Depending on its phosphorylation state, Crh exerts regulatory functions in connection with carbohydrate metabolism. So far, knowledge on phosphorylation of Crh in vivo has been limited and derived from indirect evidence. Here, we studied the dynamics of Crh phosphorylation directly by non-denaturing gel electrophoresis followed by Western analysis. The results confirm that HPrK/P is the single kinase catalyzing phosphorylation of Crh in vivo. Accordingly, phosphorylation of Crh is triggered by the carbon source as observed

previously for HPr, but with some differences. Phosphorylation of both proteins occurred during EPZ015666 cost exponential growth and disappeared upon exhaustion of the carbon source. During exponential growth, ~ 80% of the Crh molecules were phosphorylated when cells utilized a preferred carbon source. The reverse distribution, i.e. around 20% of Crh molecules phosphorylated, was obtained upon utilization of less favorable substrates. This clear-cut classification of the substrates into two groups has not previously been observed for HPr(Ser)~P formation. The likely reason for this difference is the additional

PTS-dependent phosphorylation of HPr at His15, which limits accumulation of HPr(Ser)~P. The histidine protein (HPr) of the carbohydrate : phosphotransferase system (PTS) has a dual role in Firmicutes bacteria. Megestrol Acetate In its transport function HPr delivers phosphoryl-groups from Enzyme Caspase activation I (EI) to the Enzyme II (EII) transport proteins, which phosphorylate their sugar substrates during uptake. During this phosphoryl-group transfer, HPr becomes transiently phosphorylated at residue His15. In addition, HPr also exerts important regulatory functions (Deutscher et al., 2006). It is the key player in carbon catabolite repression (CCR), which allows the bacteria to repress functions for the utilization

of secondary carbon sources when a preferred substrate is simultaneously present (Deutscher, 2008; Görke & Stülke, 2008). To be active in CCR, HPr must be phosphorylated at a different site, Ser46. HPr(Ser)~P binds the global transcriptional regulatory protein CcpA, which thereby gains DNA-binding activity (Fujita, 2009). Phosphorylation as well as de-phosphorylation of HPr at Ser46 is catalyzed by a single enzyme, the HPr kinase/phosphorylase (HPrK/P). The decision as to whether kinase or phosphorylase activity will prevail is controlled by the quality of the available carbon source. Preferred carbon sources such as glucose or fructose, which allow the fastest growth rates, activate the kinase function of HPrK/P and thereby trigger the formation of HPr(Ser)~P.