Volume 2, no. 4, [e00664-14][1], 2014. Page 1: The article title should read as given above and the word “clorobenzoic” should read “chlorobenzoic” in the abstract and main text.
[1]: /lookup/doi/10.1128/genomeA.00664-14
Abstract Background Over the past fifteen years, antibiotic resistance in the Gram-positive opportunistic human pathogen Streptococcus pneumoniae has significantly increased. Clinical isolates from patients with community-acquired pneumonia or otitis media often display resistance to two or more antibiotics. Given the need for new therapeutics, we intend to investigate enzymes of cell wall biosynthesis as novel drug targets. Alanine racemase, a ubiquitous enzyme among bacteria and absent in humans, provides the essential cell wall precursor, D-alanine, which forms part of the tetrapeptide crosslinking the peptidoglycan layer. Results The alanine racemases gene from S. pneumoniae ( alr SP ) was amplified by PCR and cloned and expressed in Escherichia coli . The 367 amino acid, 39854 Da dimeric enzyme was purified to electrophoretic homogeneity and preliminary crystals were obtained. Racemic activity was demonstrated through complementation of an alr auxotroph of E. coli growing on L-alanine. In an alanine racemases photometric assay, specific activities of 87.0 and 84.8 U mg -1 were determined for the conversion of D- to L-alanine and L- to D-alanine, respectively. Conclusion We have isolated and characterized the alanine racemase gene from the opportunistic human pathogen S. pneumoniae . The enzyme shows sufficient homology with other alanine racemases to allow its integration into our ongoing structure-based drug design project.
The structure of the catabolic alanine racemase, DadX, from the pathogenic bacterium Pseudomonas aeruginosa, reported here at 1.45 Å resolution, is a dimer in which each monomer is comprised of two domains, an eight-stranded α/β barrel containing the PLP cofactor and a second domain primarily composed of β-strands. The geometry of each domain is very similar to that of Bacillus stearothermophilus alanine racemase, but the rotation between domains differs by about 15°. This change does not alter the structure of the active site in which almost all residues superimpose well with a low rms difference of 0.86 Å. Unexpectedly, the active site of DadX contains a guest substrate that is located where acetate and propionate have been observed in the Bacillus structures. It is modeled as d-lysine and oriented such that its terminal NZ atom makes a covalent bond with C4' of PLP. Since the internal aldimine bond between the protein lysine, Lys33, and C4' of PLP is also unambiguously observed, there appears to be an equilibrium between both internally and externally reacted forms. The PLP cofactor adopts two partially occupied conformational states that resemble previously reported internal and external aldimine complexes.
A previously described regulatory mutation which abolishes expression of the extracellular nuclease of Serratia marcescens is shown to be a mutation of the Serratia recA gene. The defect in nuclease expression could be restored by introducing a plasmid carrying the recA gene of Escherichia coli. The DNA sequence of the Serratia gene is very similar to that of the E. coli gene. The putative LexA-binding site of the Serratia recA gene is almost identical to that of E. coli, along with the promoter. A similar LexA-binding site can also be found upstream of the nuclease gene. As expected from this finding, we show that nuclease expression can be induced by SOS-inducing agents such as mitomycin C. Although inducible in S. marcescens, the nuclease was expressed only at the uninduced levels in E. coli and could not be induced by mitomycin C. The extracellular chitinase and lipase were similarly affected by the mutations altering nuclease expression and were also induced by mitomycin C.
Cupriavidus sp. strain SK-4 is a bacterium capable of growing aerobically on monochlorobiphenyls and dichlorobiphenyls as the sole carbon sources for growth. Here, we report its draft genome sequence with the aim of facilitating an understanding of polychlorinated biphenyl biodegradation mechanisms.
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