Recognizing Specific Regions in the DNA Duplex

The problem of regulation in prokaryotes such as E. coli is far simpler than in complex eukaryotes because of the smaller number of genes. It seems likely that a system of this complexity (about 3,000 genes) should contain no more than 100-300 regulatory proteins because genes with related functions are often under the control of the same regulatory proteins. 

Helix-turn-helix motif. Helix 2 is the recognition helix. Individual amino acids are numbered. Residues 4, 5, 8, 9, 10, and 15 tend to be conserved in different regulatory proteins. (From Carl Branden and John Tooze, Introduction to Protein Structure, Garland Publishing, Inc., New York, 1991, p. 102.) 

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Interaction between DNA and DNA-Binding Proteins Recognizing Specific Regions in the DNA Duplex The Helix-Turn-Helix Is the Most Common Motif Found in Prokaryotic Regulatory Proteins Helix-Turn-Helix Regulatory Proteins Are Symmetrical... 

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