Sequence-based identification and characterization of mer operon-related genes from Kongsfjorden sediments

Mercury resistance is commonly associated with the mer operon, which comprises genes involved in mercury sensing, regulation, transport, and transformation. In the present study, metagenomic DNA extracted from Kongsfjorden sediments, Svalbard, was screened using a previously reported merRTΔP-targeting primer pair, followed by PCR, cloning, Sanger sequencing, and subsequent in silico analyses to characterise putative mercury-resistance-associated genetic components. A mer operon-related genetic region, designated KGmer1, comprising a regulator merR sequence, an operator-promotor (merOP) region, and two transporter merT and merP sequences, was identified. Comparative sequence analyses revealed that the identified genes shared high similarity with mer operon components reported from Pseudomonas aeruginosa. The obtained KG-merR1 gene encoded a regulator protein consisting of a DNA binding domain (DBD), dimerization helix (DH), and a metal binding domain (MBD). The transporter gene (KG-merT1) encoded a membrane protein with three transmembrane helices with dual cysteine residues associated with mercury transport. The KG-merP1 gene encoded a precursor protein with a ‘ferredoxin-like’ structural fold (βαββαβ) and a highly conserved GMTCSAC motif with two reactive cysteine residues (Cys14 and Cys17) at the metal binding site. Phylogenetic and structural analyses further demonstrated strong conservation of sequence and structural features with previously characterized mer operon components. The identified genetic region provides sequence-based evidence for the occurrence of putative mer-associated mercury-resistance components in Kongsfjorden sediments. This study provides molecular insights into the occurrence of mer operon-related genetic components in Arctic fjord sediments.