The interaction of T4 phage-encoded anti-sigma factor, asiA, and Escherichia coli ς70 was studied by using the yeast two-hybrid system. Truncation of ς70 to identify the minimum region involved in the interaction showed that the fragment containing amino acid residues proximal to the C terminus (residues 547 to 603) was sufficient for complexing to asiA. Studies also indicated that some of the truncated C-terminal fragments (residues 493 to 613) had higher affinity for asiA as judged by the increased β-galactosidase activity. It is proposed that the observed higher affinity may be due to the unmasking of the binding region of asiA on the sigma protein. Advantage was taken of the increased affinity of truncated ς70 fragments to asiA in designing a coexpression system wherein the toxicity of asiA expression in E. coli could be neutralized and the complex of truncated ς70 and asiA could be expressed in large quantities and purified
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