Novel mode of transcription regulation by SdiA, an Escherichia coli homologue of the quorum-sensing regulator.

SdiA, an Escherichia coli homologue of the quorum-sensing regulator, controls the expression of the ftsQAZ operon for cell division. Transcription of ftsQ is under the control of two promoters, upstream ftsQP2 and downstream ftsQP1, which are separated by 125 bp. SdiA activates transcription from ftsQP2 in vivo. Here, we demonstrate that SdiA facilitates the RNA polymerase binding to ftsQP2 and thereby stimulates transcription from P2. Gel shift and DNase I footprinting assays indicated that SdiA binds to the ftsQP2 promoter region between -51 and -25 with respect to the P2 promoter. Activation of ftsQP2 transcription by SdiA was observed with a mutant RNA polymerase containing a C-terminal domain (CTD)-deleted alpha-subunit (alpha 235) but not with RNA polymerase containing sigma(S) or a CTD-deleted sigma(D) (sigma(D)529). In good agreement with the transcription assay, no protection of P2 was observed with the RNA polymerase holoenzymes, E sigma(S) and E sigma(D)529. These observations together indicate that: (i) SdiA supports the RNA polymerase binding to ftsQP2; and (ii) this recruitment of RNA polymerase by SdiA depends on the presence of intact sigmaCTD. This is in contrast to the well-known mechanism of RNA polymerase recruitment by protein-protein contact between class I factors and alpha CTD. In addition to the P2 activation, SdiA inhibited RNA polymerase binding to the ftsQP1 promoter and thereby repressed transcription from P1. Gel shift assays indicate weak binding of SdiA to the P1 promoter region downstream from -13 (or +112 with respect to P2). Neither alpha CTD nor sigma CTD are required for this inhibition. Thus, the transcription repression of P1 by SdiA may result from its competition with the RNA polymerase in binding to this promoter.