Hemerythrins are widespread and conserved for methanotrophic guilds

Abstract

Hemerythrins (Hrs) are non-heme oxygen-binding proteins which act as carriers of di-oxygen. Hrs were earlier thought to be present only in marine invertebrates. Bacteriohemerythrins were discovered for the first time in two prokaryotes, of which one was a methanotroph, Methylococcus capsulatus and later found to be present in many bacterial and archaeal families. The role of hemerythrin in Methylococcus had been predicted to be in transporting oxygen to the particulate methane monooxygenase enzyme (pMMO). In our present study, we investigated the presence of Hrs as well as Hr-like domains in the sequenced genomes of methanotrophs. We found that most of the methanotrophs (80%) have a single and up to three variants of Hr and a few possess additional or exclusively Hr containing domain/s. Phylogenetic analysis showed that Hr from all the methanotrophic guilds (i.e. alpha-, gamma-proteobacterial and Verrucomicrobial) grouped together. The phylogeny of Hr showed two main clusters and two small branches. The phylogeny of pMMO and Hr Cluster I showed congruence. Three novel methanotrophic genera isolated from rice fields showed the presence of multiple and divergent copies of Hrs. Hr and Hr domain was found to be absent in Methylocella silvestris a facultative methanotroph which lacks pMMO and intracytoplasmic membranes (ICM). Single domain Hr may have relevance with the pink or brown coloration seen in methanotrophs as almost all of the colored methanotrophs showed single domain Hr in their genomes.