Sugar uptake and carbon catabolite repression in Bacillus megaterium strains with inactivated ptsHI

We have determined the role played by the phosphoenolpyruvate:sugar phosphotransferase system (PTS) in carbon catabolite repression (CCR) of xylose utilization in Bacillus megaterium. For that purpose we have cloned, sequenced and inactivated the genes ptsH and ptsl of B. megaterium, encoding HPr and EI of the PTS, respectively. While glucose uptake of a ptsHI mutant is not affected at 12.5 mM of glucose, CCR of the xyl operon is reduced in this mutant from 16-fold to 3-fold. This may be attributed to the loss of the corepressor of CcpA, HPr(Ser-P), or could result from the slower growth rate of the mutant. In contrast, CCR exerted by fructose or mannitol is completely abolished. We conclude that glucose triggers additional mechanisms of CCR than fructose or mannitol. The remaining 3-fold glucose repression is relieved in a strain in which ptsHI and glk, encoding glucokinase, are inactivated. This result indicates that glucose metabolism is necessary for CCR. The ability of the ptsHI mutant to take up glucose suggests the existence of a second, non-PTS glucose uptake system. The Km and vmax values of this transporter ranged between 2 and 5 mM and 154 to 219 nmol/[(mg protein)*min], respectively.