Sequence analysis and specificity of distinct types of menaquinone methyltransferases indicate the widespread potential of methylmenaquinone production in bacteria and archaea

Menaquinone (MK) serves as an essential membranous redox mediator in various electron transport chains of aerobic and anaerobic respiration. In addition, the composition of the quinone/quinol pool has been widely used as a biomarker in microbial taxonomy. The HemN-like class C radical SAM methyltransferases (RSMTs) MqnK, MenK and MenK2 have recently been shown to facilitate specific menaquinone methylation reactions at position C-8 (MqnK/MenK) or C-7 (MenK2) to synthesize 8-methylmenaquinone, 7-methylmenaquinone and 7,8-dimethylmenaquinone. However, the vast majority of protein sequences from the MqnK/MenK/MenK2 family belong to organisms, whose capacity to produce methylated menaquinones has not been investigated biochemically. Here, representative putative menK and menK2 genes from Collinsella tanakaei and Ferrimonas marina were individually expressed in Escherichia coli (wild-type or ubiE deletion mutant) and the corresponding cells were found to produce methylated derivatives of the endogenous MK and 2-demethylmenaquinone. Cluster and phylogenetic analyses of 828 (methyl)menaquinone methyltransferase sequences revealed signature motifs that allowed to discriminate enzymes of the MqnK/MenK/MenK2 family from other radical SAM enzymes and to identify C-7-specific menaquinone methyltransferases of the MenK2 subfamily. This study will help to predict the methylation status of the quinone/quinol pool of a microbial species (or even a microbial community) from its (meta)genome and contribute to the future design of microbial quinone/quinol pools in a Synthetic Biology approach.