There is a wealth of information on the genetic regulation and biochemical properties of bacterial C 4 -dicarboxylate transport systems. In sharp contrast, there are far fewer studies describing the transport and assimilation of C 5 -dicarboxylates among bacteria. In an effort to better our understanding on this subject, we identified the structural and regulatory genes necessary for the utilization of ␣-ketoglutarate (␣-KG) in Pseudomonas aeruginosa PAO1. The PA5530 gene, encoding a putative dicarboxylate transporter, was found to be essential for the growth of P. aeruginosa PAO1 on both ␣-KG and glutarate (another C 5 -dicarboxylate). Metabolite analysis confirmed that the PA5530 gene was necessary for the uptake of extracellular ␣-KG. Like other substrate-inducible transporter genes, expression of the PA5530 gene was induced by extracellular C 5 -dicarboxylates. It was later found that the expression of the PA5530 gene was driven solely by a ؊24/؊12 promoter recognized by the alternative sigma factor RpoN. Surprisingly, the enhancer binding protein MifR, which is known to have an essential role in biofilm development, was required for the expression of the PA5530 gene. The MifR protein is homologous to other transcriptional regulators involved in dicarboxylate assimilation, suggesting that MifR might interact with RpoN to activate the expression of the PA5530 gene in response to extracellular C 5 -dicarboxylates, especially ␣-KG. The results of this study provide a framework for exploring the assimilation of ␣-KG in other pseudomonads.
Regular crystalline surface layers (S-layers) are widespread among prokaryotes and probably represent the earliest cell wall structures. S-layer genes have been found in approximately 400 different species of the prokaryotic domains bacteria and archaea. S-layers usually consist of a single (glyco-)protein species with molecular masses ranging from about 40 to 200 kDa that form lattices of oblique, tetragonal, or hexagonal architecture. The primary sequences of hyperthermophilic archaeal species exhibit some characteristic signatures. Further adaptations to their specific environments occur by various post-translational modifications, such as linkage of glycans, lipids, phosphate, and sulfate groups to the protein or by proteolytic processing. Specific domains direct the anchoring of the S-layer to the underlying cell wall components and transport across the cytoplasma membrane. In addition to their presumptive original role as protective coats in archaea and bacteria, they have adapted new functions, e.g., as molecular sieves, attachment sites for extracellular enzymes, and virulence factors.
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