Adaptation of Lactobacillus sakei to meat: a new regulatory mechanism of ribose utilization?

Lactobacillus sakei is a lactic acid bacterium commonly found on meat and m eat products. Meat is a rich substrate but contains small amounts of sugars , mainly glucose and ribose. The phosphotransferase system (PTS) is a multi enzymatic complex responsible for sugar uptake. It is also involved in the regulation of metabolism through various mechanisms (catabolite activation and repression, inducer expulsion and exclusion). The ptsHI operon of L. sa kei, encoding the general enzymes of the PTS, was studied and mutants were constructed. On ribose, these mutants grow twice as fast as the wild-type s train. This phenotype was never described in other bacteria and suggests th at the PTS regulates ribose utilization. When compared to what is known fro m the regulations involving the PTS in other bacteria, this mechanism might be new. In Bacillus subtilis and Escherichia coli, in which ribose catabol ism was investigated, ribose is transported by an ABC transporter, encoded by rbsABCD genes and then phosphorylated by the rbsK encoded ribose kinase. Ribose-5P is then metabolized through the pentose-P pathway involving xylu lose-5P phosphoketolase and acetate kinase. Whereas phosphoketolase and ace tate kinase activities remained unchanged in L. sakei ptsI mutants, ribose kinase activity and uptake were increased by a factor of 2.5 and 1.5, respe ctively. The target of the PTS regulation would thus be transport and/or ph osphorylation of ribose. The gene cluster encoding a ribose transporter, ri bose kinase and a regulator was cloned and sequenced. In L. sakei no gene e ncoding RbsA, RbsB or RbsC could be found. However, rbsD was present as wel l as a new gene rbsU, encoding a protein homologous to a glucose transporte r responsible for facilitated diffusion of glucose. The rbsUDK operon is in duced by ribose via the regulator RbsR encoded by rbsR located downstream o f rbsUDK. In ptsI mutants, this operon was not overexpressed on ribose. Thi s shows that the regulation of ribose utilization is not a transcriptional regulation. Upstream from the rbs operon, a gene encoding acetate kinase (a ckA) was found. In other bacteria in which these genes were identified, ack A and the rbs operon are not linked. Moreover, in B. subtilis, ackA is regu lated by catabolite activation whereas the rbs operon is repressed by catab olite repression, two mechanisms involving the PTS. In L. sakei, ackA and t he rbs operon are adjacent on the chromosome and they are not regulated as described above. We propose that in this species, ribose and glucose utiliz ation is regulated in a different way, allowing L. sakei to catabolize both glucose and ribose, the sole sugars present in meat.