Abstract
Pimeloyl-acyl carrier protein (ACP) methyl ester esterase catalyzes the last biosynthetic step of the pimelate moiety of biotin, a key intermediate in biotin biosynthesis. The paradigm pimeloyl-ACP methyl ester esterase is the BioH protein of Escherichia coli that hydrolyses the ester bond of pimeloyl-ACP methyl ester. Biotin synthesis in E. coli also requires the function of the malonyl-ACP methyltransferase gene (bioC) to employ a methylation strategy to allow elongation of a temporarily disguised malonate moiety to a pimelate moiety by the fatty acid synthetic enzymes. However, bioinformatics analyses of the extant bacterial genomes showed that bioH is absent in many bioC-containing bacteria. The genome of the Gram-negative bacterium, Moraxella catarrhalis lacks a gene encoding a homolog of any of the six known pimeloyl-ACP methyl ester esterase isozymes suggesting that this organism encodes a novel pimeloyl-ACP methyl ester esterase isoform. We report that this is the case. The gene encoding the new isoform, called btsA, was isolated by complementation of an E. coli bioH deletion strain. The requirement of BtsA for the biotin biosynthesis in M. catarrhalis was confirmed by a biotin auxotrophic phenotype caused by deletion of btsA in vivo and a reconstituted in vitro desthiobiotin synthesis system. Purified BtsA was shown to cleave the physiological substrate pimeloyl-ACP methyl ester to pimeloyl-ACP by use of a Ser117-His254-Asp287 catalytic triad. The lack of sequence alignment with other isozymes together with phylogenetic analyses revealed BtsA as a new class of pimeloyl-ACP methyl ester esterase. The involvement of BtsA in M. catarrhalis virulence was confirmed by the defect of bacterial invasion to lung epithelial cells and survival within macrophages in the ΔbtsA strains. Identification of the new esterase gene btsA exclusive in Moraxella species that links biotin biosynthesis to bacterial virulence, can reveal a new valuable target for development of drugs against M. catarrhalis.
Highlights
Moraxella catarrhalis is a Gram-negative, human-restricted opportunistic bacterial pathogen that colonizes the upper and lower respiratory tracts
Biotin is synthesized de novo from the seven-carbon α,ω-dicarboxylate intermediate, pimelate, which is esterified with either CoA or acyl carrier protein (Lin et al, 2010; Cronan, 2014)
Analysis of the M. catarrhalis genome revealed that it contains four genes encoding homologs of E. coli BioA, BioF, BioC, and BioD that are clustered in an operon, but the bioB gene is remote from the cognate bioAFCD operon (Figure 1A)
Summary
Moraxella catarrhalis is a Gram-negative, human-restricted opportunistic bacterial pathogen that colonizes the upper and lower respiratory tracts. M. catarrhalis can be carried asymptomatically (known as carriage), but can causes otitis media in children and lower respiratory tract infections in adults with chronic obstructive pulmonary disease (Verduin et al, 2002) It is commonly found in a polymicrobial community with other pathogens such as Streptococcus pneumoniae and Haemophilus influenzae. BioG (Shi et al, 2016), BioK (Shapiro et al, 2012), BioJ (Feng et al, 2014), and BioV (Bi et al, 2016), have been discovered in bacteria that encode BioC but not BioH (Rodionov et al, 2002) All these esterases have been demonstrated to catalyze the cleavage of pimeloyl-ACP methyl ester in vitro and to rescue biotin synthesis in the E. coli bioH strains
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