Functional analysis of the tdcABC promoter of Escherichia coli: roles of TdcA and TdcR.

The efficient expression of the tdc operon of Escherichia coli requires the products of two regulatory genes, tdcR and tdcA. We have identified the transcription site of tdcR by primer extension mapping and established the translation start site of TdcR by mutational analysis of its reading frame. In a tdcR tdcABC deletion strain, tdcR+ promoted high-level LacZ expression from a lambda tdcAB-lacZ lysogen and mutations introduced in tdcR resulted in a greater than sixfold decrease in LacZ level. In-frame deletions of tdcA also reduced LacZ expression, and chromosomal and plasmid-borne tdcA+ increased the LacZ level in tdcA mutant lysogens. Interestingly, multicopy tdcA+ plasmids introduced into tdcR mutant strains completely restored tdc expression. In separate experiments we found that mutations in the tdc promoter DNA around positions -70, -140, and -175 greatly reduced tdc expression relative to that for the wild-type promoter and the tdcP mutation around -175 prevented multicopy tdcA+ from rescuing tdcR mutants. Furthermore, competition experiments revealed that a wild-type promoter fragment encompassing the -175 region cloned into a plasmid reduced tdc expression by titrating TdcA in vivo, and this effect was reversed with excess TdcA. These results suggest that in tdcR+ cells TdcR interacts with tdcP and/or TdcA to enhance tdc transcription whereas in tdcR mutant cells a new tdcP-TdcA complex around -175 in the native promoter bypasses the requirement for TdcR. On the basis of the accumulated data summarized here and elsewhere we propose that multiple transcription factors enhance tdc operon expression by bending and looping of the promoter DNA to form an active transcription complex.

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