The role of horizontal gene transfer in the resistance to copper and nickel in Streptomyces species: a phylogenetic analysis

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Over the last few decades, there has been increasing contamination of environments and ecosystems with heavy metals, which have effects upon the health of most eukaryotic life forms. One of the predominant microorganisms residing in environments exposed to pollutant metals, such as copper and nickel, are Streptomyces species, and these have developed various mechanisms of resistance to tolerate and achieve bioremediation. This research aimed to explore whether four metal resistance genes for Nickel and Copper had been horizontally transferred by Streptomyces species. A total of 29 Streptomyces species were selected for evaluation and the amino acid sequences for Nickel resistance genes (HoxN and NikB) and Copper resistance genes (CopA and AcrD) were obtained from BacMet , whilst the amino acid sequence for RpoB was sought from StrepDB .The multiple sequence alignments were used to construct maximum likelihood (ML) trees using PhyML. The results showed that the majority of Streptomyces species (23-24/29) had acquired the acrD and copA genes, whilst the presence of genes conferring Nickel resistance was more variable, nikB and hoxN were present in (29-13/29) species respectively. However, there was phylogenetic evidence to suggest that these genes had been transferred horizontally between species given that existence was predominant among the most recently diverged species. The results of this study favouring horizontal transfer suggest that there was a minimal risk of false-positives or false-negatives. Future research should explore whether other metal resistance genes are transferred horizontally, as this may lead to achieve large scale bioremediation and in turn, protecting the health of eukaryotic life.