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A LexA mutant repressor with a relaxed inter‐domain linker

The LexA protein is part of a large family of prokaryotic transcriptional repressors that contain an amino‐terminal DNA binding domain and a carboxy‐terminal dimerization domain. These domains are separated by a linker or hinge region, which is generally considered to be rather flexible and unconstrained. So far, no structure of any of the full‐length repressors is available. Here we show that a mutant LexA repressor harboring several point mutations in the hinge region gets sensitive to trypsin and Glu‐C cleavage over a segment of at least 20 amino acids, whereas the LexA wild‐type hinge region is resistant to these proteases. These data are not compatible with the hypothesis of an fully flexible and/or unstructured inter‐domain linker and suggest that the LexA hinge region is, in fact, constrained by contacts with the carboxy‐terminal domain and/or a fairly stable local structure of the linker region.