Phase variation in Salmonella flagellin genes. (a) In one orientation, fljB is expressed along with a repressor protein (product of the fljA gene) that turns off transcription of the fliC gene; the result is production of the flagellin FljB. (b) In the opposite orientation, the fljA and fljB genes cannot be transcribed, and only the fliC gene is expressed, producing flagellin FliC. Salmonella can flip between these two flagellin-