TdcA, a transcriptional activator of the tdcABC operon of Escherichia coli, is a member of the LysR family of proteins.

The tdcB and tdcC genes of the tdcABC operon of Escherichia coli encode threonine dehydratase and a threonine-serine permease, respectively. These proteins are involved in transport and metabolism of threonine and serine during anaerobic growth. In this study, we functionally characterized tdcA, which encodes a 35 kDa polypeptide consisting of 312 amino acid residues. Non-polar and partially polar mutations introduced into tdcA drastically reduced the expression of the genes down-stream from tdcA. Complementation studies using single-copy chromosomal integrants of a tdcB-lacZ fusion harboring an in-frame deletion of tdcA with chromosomal or plasmid-borne tdcA+ in trans showed complete restoration of tdc operon expression in vivo. The amino acid sequence at the amino-terminal end of TdcA revealed a significant homology to the helix-turn-helix motifs of typical DNA binding proteins. Sequence alignment of TdcA with LysR also showed considerable sequence similarity throughout their entire lengths. Our results suggest that TdcA is related to the LysR family of proteins by common ancestry and, based on its functional role in tdc expression, belongs to the LysR family of transcriptional activators.