The CII protein of the temperate bacteriophage l is the decision-making factor that determines the viral lytic/lysogenic choice. It is a homotetrameric transcription activator that recognizes and binds specific direct repeat sequences TTGCN6TTGC in the λ genome. The quaternary structure of CII is held by a four-helix bundle. It is known that the tetrameric organization of CII is necessary for its activity, but the molecular mechanism behind this requirement is not known. By specific site-directed mutagenesis of hydrophobic residues in the α4 helix of CII that constitutes the fourhelix bundle, we found that residues leu70, val74 and leu78 were crucial for maintaining the tetrameric structure of the protein. When any of these residues was substituted by a polar one, CII lost its activity and failed to promote lysogeny. This loss of activity was accompanied by the inability of CII to form tetramers, to bind DNA or to activate transcription.
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