3), phosphorylation of Crh and HPr at Ser46 was strongly inhibited in the untreated cells (no additional glucose added) when IWR-1 chemical structure growth ceased, i.e. after 9 h incubation (Fig. 4b, top panels). In contrast, much higher amounts of Crh~P and HPr(Ser)~P were detectable at that time (9 h) in the cells that were supplemented with additional glucose (Fig. 4b, compare lanes 3 and 10 in the top and bottom panels). This result unequivocally shows that exhaustion of the carbon source glucose prevents phosphorylation of Crh and HPr
by HPrK/P when cells enter the stationary growth phase. In this work, we analyzed the dynamics of phosphorylation of Crh in response to different nutritional conditions in vivo. Previous in vitro studies suggested that Crh becomes (de)-phosphorylated by HPrK/P at residue Ser46 like its homolog HPr, but whether this also applied to in vivo conditions was not clear. Our data confirm that
HPrK/P is actually the kinase responsible for phosphorylation of Crh in vivo (Fig. 2). Thus, one might expect a similar dynamics Midostaurin of phosphorylation of Crh and HPr at their Ser46-sites. Overall, this was indeed the case, but with some remarkable deviations. As expected, both Crh~P and HPr(Ser)~P levels decreased drastically or even disappeared when cells entered the stationary growth phase (Fig. 3). Exhaustion of the carbon source is responsible for accumulation of the non-phosphorylated proteins in this growth phase (Fig. 4). Consequently, stationary cells are released from CCR and primed for the uptake and utilization of alternative carbon sources. The degree to which Crh became phosphorylated during exponential growth depended on the quality of the carbon isometheptene source. The various substrates could be classified into two
distinct groups, triggering the formation of either low or very high levels of Crh~P (Fig. 2). Such a splitting of the carbon sources into two distinct groups has not been observed previously in the formation of HPr(Ser)~P. In this case, a more gradual transition between the various substrates was detected (Singh et al., 2008). Nonetheless, the carbon sources that trigger either very low or very high levels of phosphorylation are the same for both proteins. Only a little Crh~P and HPr(Ser)~P is formed (Fig. 2; Singh et al., 2008) when cells utilize succinate, ribose or gluconate. Consequently, these gluconeogenic carbon sources cause no or only weak CCR (Singh et al., 2008). Except for gluconate, these substrates also yield slower growth rates in comparison with the other tested substrates (Fig. 2a; Singh et al., 2008). In contrast, high Crh~P as well as HPr(Ser~P) levels were detectable when a substrate of the PTS (glucose, fructose, mannitol, salicin, sucrose), sorbitol or glycerol was the carbon source (Fig. 2; Singh et al., 2008). Accordingly, all these sugars, which exert a strong CCR, enter the upper branch of the EMP pathway directly (Singh et al., 2008).