Sequence homology between Lac and Gal repressors and three sugar-binding periplasmic proteins

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Abstract
Many proteins consist of several independent folding units or domains, each specifying a different function1. Repressor proteins such as Lac or λ cI carry small N-terminal domains which recognize DNA sequences and larger C-terminal domains which are required for effector recognition and/or oligomerization2–4. The native periplasmic metabolite-binding proteins consist of short membrane-recognizing signal sequences5 and larger C-terminal metabolite-binding domains which also recognize membrane-bound proteins involved in transport6 and chemotaxis7. The DNA-recognizing domains of many repressors are homologous8–12, as are the sugar-recognizing periplasmic proteins13. Here I demonstrate that the sugar-binding domains of the Lac and Gal repressors are homologous with the sugar-binding domains of three periplasmic proteins.