Two helix DNA binding motif of CAP found in lac repressor and gal repressor

Abstract

Comparison of both the DNA and protein sequences of catabolite gene activator protein (CAP) with the sequences of lac gal repressor. shows significant homologies between a sequence that forms a two α-helix motif in CAP and sequences user the amino terminus of both repressor. This two-helix motif is thought to be involved in specific DNA sequence recognition bydCAP. The region in lac repressor to which CAP is homologous contains many i mutations that are defective in DNA binding. Less significant sequence homologies between CAP and phage repressors and activators are also shown. The amino acid residues that are critical to the formation of the two-helix motif are conserved, while those residues expected to interact with DNA are variable. These observations suggest that the lac and gal repressors also have a two α-helix structural motif which is involved in DNA binding and that this two helix motif may be generally found in many bacterial and phage repressors. We conclude that one major mechanism by which proteins can recognize specific base sequences in double stranded DNA is via the amino acid side chains of α-helices fitting into the major groove of B-DNA. © 1982 IRL Press Limited.