Abstract
BioH is an α/β-hydrolase required for synthesis of the pimelate moiety of biotin in diverse bacteria. The bioH gene is found in different genomic contexts. In some cases (e.g., Escherichia coli) the gene is not located within a biotin synthetic operon and its transcription is not coregulated with the other biotin synthesis genes. In other genomes such as Pseudomonas aeruginosa the bioH gene is within a biotin synthesis operon and its transcription is coregulated with the other biotin operon genes. The esterases of pimelate moiety synthesis show remarkable genomic plasticity in that in some biotin operons bioH is replaced by other α/ß hydrolases of diverse sequence. The “wild card” nature of these enzymes led us to compare the paradigm “freestanding” E. coli BioH with the operon-encoded P. aeruginosa BioH. We hypothesized that the operon-encoded BioH might differ in its expression level and/or activity from the freestanding BioH gene. We report this is not the case. The two BioH proteins show remarkably similar hydrolase activities and substrate specificity. Moreover, Pseudomonas aeruginosa BioH is more highly expressed than E. coli BioH. Despite the enzymatic similarities of the two BioH proteins, bioinformatics analysis places the freestanding and operon-encoded BioH proteins into distinct clades.
Highlights
Biotin functions only when covalently attached to its cognate proteins which are involved in key metabolic carboxylation and decarboxylation reactions
Expression of the P. aeruginosa in an E. coli ∆bioH strain resulted in robust growth in biotin-free medium as expected from prior studies with various nonorthologous esterases[24, 25, 30, 31]
Note that despite its low sequence identity with E. coli BioH the P. aeruginosa protein is clearly a BioH and not one of the other pimeloyl-acyl carrier protein (ACP) methyl ester esterases because those nonorthologous proteins cannot be aligned with E. coli BioH even given very permissive alignment parameters[24, 25, 30]
Summary
Biotin functions only when covalently attached to its cognate proteins which are involved in key metabolic carboxylation and decarboxylation reactions. The pathway for synthesis of this moiety was demonstrated only recently in E. coli and consists of enzymes encoded by the bioH and bioC genes that allow the fatty acid synthesis pathway to make pimelate, a seven carbon dicarboxylic acid[9, 13] (Fig. 1a). Examples are BioK in cyanobacteria[16], BioV in Helicobacter species[24] and BioJ in Francisella species[25] This unexpected diversity argues that some of the enzymes that cleave the methyl ester of pimeloyl-ACP methyl ester may have arisen recently and may not be attuned to the low demands of biotin synthesis (E. coli growth requires only a few hundred biotin molecules per cell). To test this hypothesis we determined the relative expression levels, catalytic activities and specificities of two BioH proteins, those encoded by the freestanding E. coli bioH and the operon-encoded bioH of Pseudomonas aeruginosa PAO1
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