Characterization of a sugar-binding protein from Azospirillum brasilense mediating chemotaxis to and uptake of sugars.

Our approach to the isolation of plant-inducible bacterial genes of Azospirillum brasilense, based on the analysis of protein patterns of bacteria grown in the presence and in the absence of plant root exudates, led to the identification of an acidic 40 kDa protein. Cloning and sequencing analysis of the corresponding coding DNA region revealed the presence of two open reading frames transcribed in the same orientation. The deduced ORF1 protein, which corresponds to the 40 kDa protein, is very similar to the periplasmic ChvE protein, identified in Agrobacterium tumefaciens and involved in enhanced virulence. The deduced ORF2 protein shows homology to members of the LysR family of transcriptional regulators. The function of the ChvE-like protein in A. brasilense was investigated further. The protein, designated as SbpA (sugar binding protein A), is involved in the uptake of D-galactose and functions in the chemotaxis of A. brasilense towards several sugars, including D-galactose, L-arabinose and D-fucose. Expression of the sbpA gene requires the presence of the same sugars in the growth medium and is enhanced further in combination with carbon starvation of A. brasilense cells.