Corrected gene assignments of Escherichia coli pro- mutations

Biosynthesis of Proline

Proline was among the last biosynthetic precursors to have its biosynthetic pathway unraveled. This review recapitulates the findings on the biosynthesis and transport of proline. Glutamyl kinase (GK) catalyzes the ATP-dependent phosphorylation of L-glutamic acid. Purification of γ-GK from Escherichia coli was facilitated by the expression of the proB and proA genes from a high-copy-number plasmid and the development of a specific coupled assay based on the NADPH-dependent reduction of GP by γ-glutamyl phosphate reductase (GPR). GPR catalyzes the NADPH-dependent reduction of GP to GSA. Site directed mutagenesis was used to identify residues that constitute the active site of E. coli GK. This analysis indicated that there is an overlap between the binding sites for glutamate and the allosteric inhibitor proline, suggesting that proline competes with the binding of glutamate. The review also summarizes the genes involved in the metabolism of proline in E. coli and Salmonella. Among the completed genomic sequences of Enterobacteriaceae, genes specifying all three proline biosynthetic enzymes can be discerned in E. coli, Shigella, Salmonella enterica, Serratia marcescens, Erwinia carotovora, Yersinia, Photorhabdus luminescens, and Sodalis glossinidius strain morsitans. The intracellular proline concentration increases with increasing external osmolality in proline-overproducing mutants. This apparent osmotic regulation of proline accumulation in the overproducing strains may be the result of increased retention or recapture of proline, achieved by osmotic stimulation of the ProP or ProU proline transport systems. A number of proline analogs can be incorporated into proteins in vivo or in vitro.

The Clostridium difficile exosporium cysteine (CdeC)-rich protein is required for exosporium morphogenesis and coat assembly

Clostridium difficile is an important nosocomial pathogen that has become a major cause of antibiotic-associated diarrhea. There is a general consensus that C. difficile spores play an important role in C. difficile pathogenesis, contributing to infection, persistence, and transmission. Evidence has demonstrated that C. difficile spores have an outermost layer, termed the exosporium, that plays some role in adherence to intestinal epithelial cells. Recently, the protein encoded by CD1067 was shown to be present in trypsin-exosporium extracts of C. difficile 630 spores. In this study, we renamed the CD1067 protein Clostridium difficile exosporium cysteine-rich protein (CdeC) and characterized its role in the structure and properties of C. difficile spores. CdeC is expressed under sporulation conditions and localizes to the C. difficile spore. Through the construction of an ΔcdeC isogenic knockout mutant derivative of C. difficile strain R20291, we demonstrated that (i) the distinctive nap layer is largely missing in ΔcdeC spores; (ii) CdeC is localized in the exosporium-like layer and is accessible to IgGs; (iii) ΔcdeC spores were more sensitive to lysozyme, ethanol, and heat treatment than wild-type spores; and (iv) despite the almost complete absence of the exosporium layer, ΔcdeC spores adhered at higher levels than wild-type spores to intestinal epithelium cell lines (i.e., HT-29 and Caco-2 cells). Collectively, these results indicate that CdeC is essential for exosporium morphogenesis and the correct assembly of the spore coat of C. difficile.

Dissection of Escherichia coli glutamate 5-kinase: functional impact of the deletion of the PUA domain

Glutamate 5-kinase (G5K) catalyzes the controlling first step of the synthesis of the osmoprotective amino acid proline, which feed-back inhibits G5K. Microbial G5K generally consists of one amino acid kinase (AAK) and one PUA (named after pseudo uridine synthases and archaeosine-specific transglycosylases) domain. To investigate the role of the PUA domain, we have deleted it from Escherichia coli G5K. We show that wild-type G5K requires free Mg for activity, it is tetrameric, and it aggregates to higher forms in a proline-dependent way. G5K lacking the PUA domain remains tetrameric, active, and proline-inhibitable, but the Mg requirement and the proline-triggered aggregation are greatly diminished and abolished, respectively, and more proline is needed for inhibition. We propose that the PUA domain modulates the function of the AAK domain, opening the way to potential PUA domain-mediated regulation of G5K; and that this domain moves, exposing new surfaces upon proline binding.

Mutations in the Corynebacterium glutamicum proline biosynthetic pathway: a natural bypass of th proA step

Two chromosomal loci containing the Corynebacterium glutamicum ATCC 17965 proB and proC genes were isolated by complementation of Escherichia coli proB and proC auxotrophic mutants. Together with a proA gene described earlier, these new genes describe the major C. glutamicum proline biosynthetic pathway. The proB and proA genes, closely linked in most bacteria, are in C. glutamicum separated by a 304-amino-acid open reading frame (unk) whose predicted sequence resembles that of the 2-hydroxy acid dehydrogenases. C. glutamicum mutants that carry null alleles of proB, proA, and proC were constructed or isolated from mutagenized cultures. Single proC mutants are auxotrophic for proline and secrete delta1-pyrroline-5-carboxylate, which are the expected phenotypes of bacterial proC mutants. However, the phenotypes or proB and proA mutants are unexpected. A proB mutant has a pleiotropic phenotype, being both proline auxotrophic and affected in cell morphology. Null proA alleles still grow slowly under proline starvation, which suggests that a proA-independent bypass of this metabolic step exists in C. glutamicum. Since proA mutants are complemented by a plasmid that contains the wild-type asd gene of C. glutamicum, the asd gene may play a role in this bypass.

Multicopy suppression by asd gene and osmotic stress-dependent complementation by heterologous proA in proA mutants

Auxotrophic proA mutants of Escherichia coli were complemented by two different classes of Corynebacterium glutamicum genes. One of these was the asd gene. The E. coli asd gene also complements the same proA alleles. Complementation of proA by the asd+ gene requires a high asd dosage and the proB and the proC gene products. The reciprocal complementation pattern (asd by the proA+ gene) was not observed. This complementation appears to be due to multicopy suppression by a proline biosynthetic gene whose product was expected to play a negligible role in this pathway. The other class of complementing clones carries the C. glutamicum proA gene. Complementation of E. coli proA mutants by the C. glutamicum proA+ gene was optimal at high osmolarity.