PHOSPHOENOLPYRUVATE - CARBOHYDRATE PHOSPHOTRANSFERASE SYSTEMS OF BACTERIA

Numerous gram-negative and gram-positive bacteria take up carbohydrate s through the phosphoenolpyruvate (PEP):carbohydrate phosphotransferas e system (PTS). This system transports and phosphorylates carbohydrate s at the expense of PEP and is the subject of this review. The PTS con sists of two general proteins, enzyme I and HPr, and a number of carbo hydrate-specific enzymes, the enzymes II. PTS proteins are phosphoprot eins in which the phospho group is attached to either a histidine resi due or, in a number of cases, a cysteine residue. After phosphorylatio n of enzyme I by PEP, the phospho group is transferred to HPr. The enz ymes II are required for the transport of the carbohydrates across the membrane and the transfer of the phospho group from phospho-HPr to th e carbohydrates. Biochemical, structural, and molecular genetic studie s have shown that the various enzymes II have the same basic structure . Each enzyme II consists of domains for specific-functions, e.g., bin ding of the carbohydrate or phosphorylation. Each enzyme II complex ca n consist of one to four different polypeptides. The enzymes II can be placed into at least four classes on the basis of sequence similarity . The genetics of the PTS is complex, and the expression of PTS protei ns is intricately regulated because of the central roles of these prot eins in nutrient acquisition. In addition to classical induction-repre ssion mechanisms involving repressor and activator proteins, other typ es of regulation, such as antitermination, have been observed in some PTSs. Apart from their role in carbohydrate transport, PTS proteins ar e involved in chemotaxis toward PTS carbohydrates. Furthermore, the IL A(Glc) protein, part of the glucose-specific PTS, is a central regulat ory protein which in its nonphosphorylated form can bind to and inhibi t several non-PTS uptake systems and thus prevent entry of inducers. I n its phosphorylated form, P-IIA(Glc) is involved in the activation of adenylate cyclase and thus in the regulation of gene expression. By s ensing the presence of PTS carbohydrates in the medium and adjusting t he phosphorylation state of IIA(Glc), cells can adapt quickly to chang ing conditions in the environment. In gram-positive bacteria, it has b een demonstrated that HPr can be phosphorylated by ATP on a serine res idue and this modification may perform a regulatory function.