The Bacterial Phosphotransferase System: Structure, Function, Regulation and Evolution

Thirty-seven years ago, Kundig, Ghosh and Roseman reported the discovery of a novel sugar-phosphorylating system in Escherichia coli (Kundig et al., 1964). The unique features of this phosphotransferase system (PTS) included the use of phosphoenolpyruvate (PEP) as the phosphoryl donor for sugar phosphorylation and the presence of three essential catalytic entities, termed Enzyme I, Enzyme II and HPr (heat-stable, histidine-phosphorylatable protein). The discovery of this system provided an explanation for pleiotropic carbohydrate-negative mutants of E. coli described as early as 1949 (Doudoroff et al., 1949). In 1964, the three recognized activities of the PTS were presumed to correspond merely to three proteins. We now recognize dozens of PTS proteins in E. coli as well as hundreds of PTS proteins in other bacteria. Numerous genes encoding these proteins have been fully sequences, and their phylogenetic relationships have been defined. In 1964, a single function for the PTS, namely sugar phosphorylation, was known. Thirty-seven years later we find that this system plays roles in many surprising aspects of bacterial cellular physiology. Established primary functions of the system include sugar reception, transport and phosphorylation, whereas secondary functions include a variety of ramifications of metabolic and transcriptional regulation (Saier et al., 1989, 1994; Saier and Reizer, 1994; Stulke et al., 1998; Stulke and Hillen, 1998). Targets of regulation include (i) carbohydrate catabolic enzymes, sugar permeases and the cyclic AMP biosynthetic enzyme, adenylate cyclase, regulated allosterically by the IIAGlc PTS protein in enteric bacteria; (ii) glycogen phosphorylase, regulated by the HPr protein in E. coli, (iii) the Mlc transcription factor, regulated by the glucose-specific permease, IICBGlc in enteric bacteria; (iv) a variety of nonPTS transport systems, a sugar-phosphate phosphatase which controls the process of inducer expulsion, and the PTS itself, regulated by HPr(ser-P) in low G+C Grampositive bacteria; (v) transcriptional activators and antiterminators regulated by direct phosphorylation in both enteric and Gram-positive bacteria; and (vi) carbohydrate catabolic enzymes and permeases, also regulated by direct phosphorylation in Gram-positive bacteria. PTS auxiliary proteins such as the fructose repressor, FruR, and the Mlc transcription factor are believed to control transcription of the PTS genetic apparatus as well as of genes encoding central pathways of carbon metabolism in enteric bacteria (Plumbridge, 1999; Saier and Ramseier, 1996). These pathways include glycolysis, the Krebs cycle, electron transport, the glyoxylate shunt, gluconeogenesis, and possibly the Entner-Doudoroff pathway. Both pts and fruR mutants of Salmonella typhimurium are greatly attenuated for virulence in mice (Groisman and Saier, 1990; Saier and Chin, 1990). Genetic evidence has indicated that other processes including the net production of carbon and energy storage sources such as poly-s-hydroxybutyrate (Pries et al., 1991) and the control of σ-dependent transcription of nitrogen metabolic genes in numerous Gram-negative bacteria (Merrick and Coppard, 1989; Reizer et al., 1992) are also controlled by the PTS. Moreover, the biochemical detection of novel, functionally uncharacterized PTS proteins in bacteria as diverse as Ancalomicrobium adetum (Saier and Staley, 1977), Spirochaeta aurantia (Saier et al., 1977), Acholeplasma laidlawii (Hoischen et al., 1993), Listeria monocytogenes (Mitchell et al., 1993) and several antibiotic-producing species of Streptomyces (Titgemeyer et al., 1994) suggests the involvement of PTS proteins in cellular processes distinct from those currently recognized. It is worth noting that other families of transport systems such as the family of ATP-binding cassette (ABC)-type permeases (Higgins, 1992), and the major facilitator superfamily (Pao et al., 1998) apparently do not participate in metabolic and transcriptional regulation, at least to the extent observed for the PTS. In this PTS symposium, dedicated to the memory of Dr. Jonathan Reizer, we shall review some current research on the PTS, discuss the multifaceted structural and functional aspects of the system and attempt to provide a realistic forecast of future discoveries. The potential benefits of PTS research seem unlimited. Its study will undoubtedly advance our fundamental knowledge of molecular evolution, will contribute to our understanding of prokaryotic physiology and pathogenesis, will allow major advances in biotechnology, and will result in the development of agents capable of effectively combating harmful microorganisms.