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The CIII protein of bacteriophage lambda exhibits antiproteolytic activity against the ubiquitous metalloprotease HflB (FtsH) of Escherichia coli, thereby stabilizing the λCII protein and promoting lysogenic development of the phage. CIII also protects E. coliσ32, another substrate of HflB. We have recently shown that the protection of CII from HflB by CIII involves direct CIII–HflB binding, without any interaction between CII and CIII [Halder S, Datta AB & Parrack P (2007) J Bacteriol189, 8130–8138]. Such a mode of action for λCIII would be independent of the HflB substrate. In this study, we tested the ability of CIII to protect σ32 from HflB digestion. The inhibition of HflB-mediated proteolysis of σ32 by CIII is very similar to that of λCII, characterized by an enhanced protection by the core CIII peptide CIIIC (amino acids 14–41 of λCIII) and a lack of interaction between σ32 and CIII.