Regulation study of exopolysaccharide synthesis, exoX and exoY in Rhizobium sp. strain NGR234

Abstract

Rhizobium sp. NGR234 produces large amounts of acidic exopolysaccaride (EPS), which can be regulated by altering the relative copy numbers of two genes, exoX and exoY, implying that varied expression from the promoters of exoX and exoY may be the means by which exopolysaccharide regulation occurs in vivo during symbiosis. For this report, specific transcriptional fusions exoX and exoY to the Escherichia coli lacZ gene were constructed and used in promoter regulation studies. It was not possible to demonstrate transcriptional regulation of exoX by exoY or exoY by exoX, suggesting that the mode by which exoX and exoY regulates EPS production may be at the post-translational level. This did not negate the possibility that one or other of the promoters may be regulated by other factors. Activity of the exoY promoter was 2.5- to 5-fold higher in the background of mutant strain ANU2895. Strain ANU2895 is a Tn5 mutant of Rhizobium sp. NGR234, which produces excessive amounts of EPS. The result suggests that the gene mutated in strain ANU2895 may encode for a trans-acting factor that acts on the promoter for exoY. In addition, a series of possible influential environmental factors that may occur in a nodule environment were tested, such as the sources of carbon or nitrogen, availability of oxygen or nitrogen and variations in pH, on the activity of exoX and exoY promoters. There was no evidence that expression from these promoters is regulated by these physiological factors; and so a link, if any, between the Rhizobium exo genes and the surrounding environmental conditions remains uncertain.