Abstract
Bacterial nitrile hydratase (NHases) are important industrial catalysts and waste water remediation tools. In a global computational screening of conventional and metagenomic sequence data for NHases, we detected the two usually separated NHase subunits fused in one protein of the choanoflagellate Monosiga brevicollis, a recently sequenced unicellular model organism from the closest sister group of Metazoa. This is the first time that an NHase is found in eukaryotes and the first time it is observed as a fusion protein. The presence of an intron, subunit fusion and expressed sequence tags covering parts of the gene exclude contamination and suggest a functional gene. Phylogenetic analyses and genomic context imply a probable ancient horizontal gene transfer (HGT) from proteobacteria. The newly discovered NHase might open biotechnological routes due to its unconventional structure, its new type of host and its apparent integration into eukaryotic protein networks.
Highlights
Nitril hydratases (NHases, E.C. 4.2.1.84) catalyze the hydrolysis of nitriles to their corresponding amids [1]
To confirm the NHases membership of the identified sequences, to study the taxonomic distribution of the originating organisms and to possibly define new subgroups we constructed maximum likelihood trees of both subunits. These trees (Figure 1) confirmed that the detected sequences are NHases and show taxonomic clustering. They illustrate that all sequences – the metagenomic ones - seem to originate from bacterial species, with a large fraction of proteobacterial NHases found in the Global Ocean Sampling Expedition dataset (Table S1 and Figure S1)
As the eukaryotic NHase has a phylogenetic position within diverse bacterial NHases (Figure 1), the currently most parsimony explanation is that it resulted from an ancient horizontal gene transfer from bacteria into the choanoflagellate or a more ancient eukaryotic lineage
Summary
Nitril hydratases (NHases, E.C. 4.2.1.84) catalyze the hydrolysis of nitriles to their corresponding amids [1] Often, this reaction is part of a two-step degradation pathway and is followed by an amidase catalyzed step. [9,10] Due to their ability to selectively and efficiently hydrolyze cyano groups, NHases are heavily used in biotechnological industry e.g. for the synthesis of the essential chemicals acrylamide (30,000 tons/year [11]) and nicotinamide (.3500 tons/year [12]). Their enzymatic activities are used to remove toxic nitriles (e.g. nitrile herbicides) during waste water treatment [13]
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