fljA

fljA is a regulatory gene in Salmonella enterica that encodes a small transcriptional regulator involved in flagellar phase variation. It is part of the fljBA region, which also includes the fljB gene encoding the H2 flagellin. The fljA/fljB locus sits within the larger flagellar gene cluster on the bacterial chromosome.

The expression of fljA and its partner fljB is controlled by an invertible promoter that is flipped by the Hin recombinase. When the promoter is oriented to drive fljBA transcription, the bacterium expresses FljB (the H2 flagellin) together with FljA. FljA functions as a negative regulator of fliC, the gene encoding the alternative flagellin (H1). This repression of fliC expression ensures predominance of the H2 antigen. When the promoter flips to an orientation that drives fliC expression, FljA and FljB are no longer produced, allowing the H1 flagellin to be expressed.

This mechanism enables antigenic variation between the H1 and H2 flagellins, which can help Salmonella evade host immune responses during infection or colonization. The broader flagellar regulon, including master regulators such as FlhDC and the FliA sigma factor, provides the general transcriptional context for these genes, though FljA specifically mediates repression of fliC to enforce phase variation.

FljA has been studied primarily in Salmonella enterica serovar Typhimurium, with conservation of the basic regulatory scheme in related Salmonella serovars. While the qualitative role of FljA as a repressor of fliC is established, details of its molecular interactions and regulatory nuances continue to be explored.