A NOVEL ROLE OF IMME7 IN THE AUTOREGULATORY EXPRESSION OF THE COLE7 OPERON AND IDENTIFICATION OF POSSIBLE RNASE ACTIVE-SITES IN THE CRYSTAL-STRUCTURE OF DIMERIC IMME7

Site-specific cleavage of mRNA has been identified in vivo for the pol ycistronic colicin E7 operon (ColE7), which occurs between G and A nuc leotides located at the Asp52 codon (GAT) of the immunity gene (ceiE7) . In vitro, this specific cleavage occurs only in the presence of the ceiE7 gene product (ImmE7). The crystal structure of dimeric ImmE7 has been determined at 1.8 Angstrom resolution by X-ray crystallographic analysis. We found that several residues located at the interface of d imeric ImmE7 bear surprising resemblance to the active sites of some R Nases. These results suggest that dimeric ImmE7 may possess a novel RN ase activity that cleaves its own mRNA at a specific site and thus aut oregulates translational expression of the downstream celE7 gene as we ll as degradation of the upstream ceaE7 mRNA.