The role of flcA in regulating root colonisation by 'Azospirillum brasilense'

Abstract

'Azospirillum brasilense' are gram-negative bacteria, which are found in the rhizosphere and are able to fix nitrogen. A regulatory gene of 'A. brasilense' Sp7, flcA, controls exopolysaccharide production, flocculation, cyst formation and colonisation of plant roots. Indirectly, by controlling differentiation into cyst-like cells, flcA also controls the level of nitrogen fixation by 'Azospirillum' associated with plant roots (1, 2). The deduced translation product of flcA, a protein of 215 amino acids, shows high homology to the C-terminal domain of transcriptional regulators of the LuxR-UhpA family (2). The protein was isolated using a histidine tag and FLPC analysis indicates that the flcA protein oligomerise to produce, at least a dimer (L. Pereg-Gerk, unpublished data), as was shown to be the case with many other transcriptional regulators. The N-terminus of flcA contains a conserved aspartate, which suggests its activity may be regulated by kinase phosphorylation as shown for environmentally controlled two component response regulators (3). The extreme C-terminus of flcA contains a helix-turn-helix motif, which is a DNA binding motif of transcriptional regulators. Therefore, flcA is most likely a part of a regulatory chain acting in response to environmental stimuli. Assuming that flcA is a transcriptional regulator, it will bind to target genes on the 'Azospirillum' genome. Thus, we are looking for genomic DNA fragments that may contain target sequences for flcA. Identifying such sequences will assist us in isolating genes regulated by flcA.