Compatible solutes are small organic molecules that are involved in the acclimation to various stresses such as temperature and salinity. Marine or moderate halotolerant cyanobacteria accumulate glucosylglycerol, while cyanobacteria with low salt tolerance (freshwater strains) usually accumulate sucrose or trehalose as the main compatible solutes. The screening of the genome of the marine, unicellular N2-fixing cyanobacterium Crocosphaera watsonii WH8501 revealed that instead of genes for glucosylglycerol biosynthesis, a fusion protein for the synthesis of trehalose was found that displayed similarities to trehalose-phosphate-synthase and -phosphatase (OtsAB pathway) from enterobacteria. Accordingly, cells of Crocosphaera showed salt-stimulated expression of the otsAB gene as well as a salt-dependent trehalose accumulation. The biochemical characterization of recombinant full-length OtsAB and truncated OtsB versions revealed that the otsAB gene in Crocosphaera encodes for an active trehalose-phosphate-synthase/phosphatase fusion protein. Genes coding for such proteins were not found in the genomes of other cyanobacteria but were present in many other, non-related marine bacteria, suggesting that otsAB might have been acquired by lateral gene transfer into the Crocosphaera genome.
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