An aerobic link between lignin degradation and C1 metabolism: growth on methoxylated aromatic compounds by members of the genus Methylobacterium

Microorganisms faces many barriers in the degradation of the polycyclic aromatic polymer lignin, one of which is an abundance of methoxy substituents. Demethoxylation of lignin-derived aromatic monomers in aerobic environments releases formaldehyde, a potent cellular toxin that organisms must eliminate in order to further degrade the aromatic ring. Here we provide the first comprehensive description of the ecology and evolution of the catabolism of methoxylated aromatics in the genus Methylobacterium, a plant-associated genus of methylotrophs capable of using formaldehyde for growth. Using comparative genomics, we found that the capacity for aromatic catabolism is ancestral to two clades, but has also been acquired horizontally by other members of the genus. Through laboratory growth assays, we demonstrated that several Methylobacterium strains can grow on p-hydroxybenzoate, protocatechuate, vanillate, and ferulate; furthermore, whereas non-methylotrophs excrete formaldehyde as a byproduct during growth on vanillate, Methylobacterium do not. Finally, we surveyed published metagenome data to find that vanillate-degrading Methylobacterium can be found in many soil and rhizosphere ecosystems but is disproportionately prominent in the phyllosphere, and the most highly represented clade in the environment (the root-nodulating species M. nodulans) is one with few cultured representatives.

The Physiological Contribution ofAcinetobacter PcaK, a Transport System That Acts upon Protocatechuate, Can Be Masked by the Overlapping Specificity of VanK

ABSTRACT VanK is the fourth member of the ubiquitous major facilitator superfamily of transport proteins to be identified that, together with PcaK, BenK, and MucK, contributes to aromatic catabolism inAcinetobacter sp. strain ADP1. VanK and PcaK have overlapping specificity for p-hydroxybenzoate and, most clearly, for protocatechuate: inactivation of both proteins severely impairs growth with protocatechuate, and the activity of either protein alone can mask the phenotype associated with inactivation of its homolog. Furthermore, vanK pcaK double-knockout mutants appear completely unable to grow in liquid culture with the hydroaromatic compound quinate, although such cells on plates convert quinate to protocatechuate, which then accumulates extracellularly and is readily visible as purple staining. This provides genetic evidence that quinate is converted to protocatechuate in the periplasm and is in line with the early argument that quinate catabolism should be physically separated from aromatic amino acid biosynthesis in the cytoplasm so as to avoid potential competition for intermediates common to both pathways. Previous studies of aromatic catabolism inAcinetobacter have taken advantage of the ability to select directly strains that contain a spontaneous mutation blocking the β-ketoadipate pathway and preventing the toxic accumulation of carboxymuconate. By using this procedure, strains with a mutation in structural or regulatory genes blocking degradation of vanillate,p-hydroxybenzoate, or protocatechuate were selected. In this study, the overlapping specificity of the VanK and PcaK permeases was exploited to directly select strains with a mutation in eithervanK or pcaK. Spontaneous mutations identified in vanK include a hot spot for frameshift mutation due to contraction of a G6 mononucleotide repeat as well as point mutations producing amino acid substitutions useful for analysis of VanK structure and function. Preliminary second-site suppression analysis using transformation-facilitated PCR mutagenesis in one VanK mutant gave results similar to those using LacY, the prototypic member of the major facilitator superfamily, consistent with the two proteins having a similar mechanism of action. The selection for transport mutants described here for Acinetobacter may also be applicable to Pseudomonas putida, where the PcaK permease has an additional role in chemotaxis.