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 )dcAB-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.The inducible threonine dehydratase operon tdcABC of Escherichia coli (6) is implicated in transport and metabolism of threonine and serine during anaerobic growth to provide a source of metabolic energy. The operon contains two structural genes, tdcB and tdcC, encoding, respectively, the biodegradative threonine dehydratase (EC 4.2.1.16) catalyzing the dehydration of L-threonine and L-serine to ammonia and the corresponding ox-keto acids and a membrane-associated Lthreonine-L-serine permease (2, 6, 23). The regulatory gene tdcA, proximal to the tdc promoter, encodes a protein homologous to the LysR family of transcriptional activators and is required for maximal induction of the tdc operon (3, 5). Upstream of tdcABC and transcribed in the opposite orientation is tdcR, which specifies a small protein that is also required for efficient expression of the tdc genes (19). The entire 6.3-kb E. coli DNA fragment harboring the tdc genes and their flanking sequences has been sequenced (18) and mapped at min 68.3, or at kb 3330, on the E. coli chromosome with clockwise direction of transcription (17,20).In addition to the operon-specific transcriptional activators TdcR and TdcA, several other gene products also influence tdc operon expression in vivo. The cyclic-AMP-catabolite gene activator protein (CAP) complex and integration host factor (I...

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