Phase variation of H. influenzae fimbriae: transcriptional control of two divergent genes through a variable combined promoter region.

The expression of H. influenzae fimbriae is subject to reversible phase variation between three expression levels. This phenomenon is controlled at the transcriptional level of two divergently orientated genes, hifA and hifB, encoding the major fimbrial subunit and the fimbrial chaperone, respectively. The hifA and hifB promoter regions were found to be clustered through an almost complete divergent overlap with a variable DNA backbone of repetitive TA units. Variation in the number of units changes the normally strictly constrained spacing between the -35 and -10 sequences and controls the bidirectional transcription initiation, thus forming a novel mechanism directing multiple gene transcription.