Genome sequencing and heterologous expression of antiporters reveal alkaline response mechanisms of Halomonas alkalicola

Halomonas alkalicola CICC 11012s is an alkaliphilic and halotolerant bacterium isolated from a soap-making tank (pH > 10) from a household-product plant. This strain can propagate at pH 12.5, which is fatal to most bacteria. Genomic analysis revealed that the genome size was 3,511,738 bp and contained 3295 protein-coding genes, including a complete cell wall and plasma membrane lipid biosynthesis pathway. Furthermore, four putative Na + /H + and K + /H + antiporter genes, or gene clusters, designated as HaNhaD , HaNhaP , HaMrp and HaPha , were identified within the genome. Heterologous expression of these genes in antiporter-deficient Escherichia coli indicated that HaNhaD, an Na + /H + antiporter, played a dominant role in Na + tolerance and pH homeostasis in acidic, neutral and alkaline environments. In addition, HaMrp exhibited Na + tolerance; however, it functioned mainly in alkaline conditions. Both HaNhaP and HaPha were identified as K + /H + antiporters that played an important role in high alkalinity and salinity. In summary, genome analysis and heterologous expression experiments demonstrated that a complete set of adaptive strategies have been developed by the double extremophilic strain CICC 11012s in response to alkalinity and salinity. Specifically, four antiporters exhibiting different physiological roles for different situations worked together to support the strain in harsh surroundings.