Characterization and spontaneous mutation of a novel gene, polE, involved in pellicle formation in Acetobacter tropicalis SKU1100

Abstract

Acetobacter tropicalis SKU1100 produces a pellicle polysaccharide, consisting of galactose, glucose and rhamnose, which attaches to the cell surface. This strain forms two types of colony on agar plates: a rough-surfaced colony (R strain) and a mucoid smooth-surfaced colony (S strain). The R strain forms a pellicle, allowing it to float on the medium surface in static culture, while the S strain does not. The pellicle is an assemblage of cells which are tightly associated with capsular polysaccharides (CPS) on the cell surface. In this study, a gene required for pellicle formation by the R strain was investigated by transposon mutagenesis using Tn10. The resulting mutant, designated Pel-, has a smooth-surfaced colony and a defect in pellicle formation, as for the S strain. The mutant produced polysaccharide which was instead secreted into the culture medium as extracellular polysaccharide (EPS). An ORF was identified at the Tn10 insertion site, designated polE, upstream of which polABCD genes were also found. The deduced amino acid sequences of polABCD showed a high level of homology to those of rfbBACD which are involved in dTDP-rhamnose synthesis, whereas polE had a relatively low level of homology to glycosyltransferase. In this study a polB (rfbA) disruptant was also prepared, which lacked both CPS and EPS production. A plasmid harbouring the polE or polB genes could restore pellicle formation in the Pel(-) mutant and S strains, and in the DeltapolB mutant, respectively. Thus both polE and polB are evidently involved in pellicle formation, most likely by anchoring polysaccharide to the cell surface and through the production of dTDP-rhamnose, respectively. The Pel- and DeltapolB mutants were unable to grow in static culture and became more sensitive to acetic acid due to the loss of pellicle formation. Additionally, this study identified the mutation sites of several S strains which were spontaneously isolated from the original culture and found them to be concentrated in a sequence of 7 C residues in the coding sequence of polE, with the deletion or addition of a single C nucleotide.