Phage AR1, previously known to infect Escherichia coli O157:H7 with high specificity, was further characterized for its genetic properties. The phage DNA sequences including capsid genes and a putative α-glucosyltransferase gene (α-gt) have been deduced. These sequences are conservative but not identical to those of T4 phage. However, a nonessential gene, SegD, organized within the capsid gene cluster of T4 is missing in the corresponding region of AR1 genome, and this characteristic has not been observed among T-even related phages. The difference between AR1 and T4 was further exemplified by their distinct host ranges. Strains of E. coli O157:H7 collected from different sources were permissive to AR1 but resistant to T4 that normally infects K-12 strains of E. coli through contact with the outer membrane protein OmpC. Thus, the O157:H7 strains must have a varied OmpC. Indeed, the OmpC sequence of O157:H7 strains was proved to differ from that of K-12 strains by a total of 15 amino acid substitutions and two gaps (a five-residue deletion and a four-residue insertion). The OmpC molecules are relatively conserved across the gram-negative bacteria, and this is the first time OmpC divergence has been found within the same E. coli species. Since OmpC is located in the outer membrane and its expression is regulated by environmental conditions, alteration of the structure in pathogenic O157:H7 strains may have biological significance.

The HIV pol sequentially encodes protease (PR), reverse transcriptase (RT), and integrase (IN) from the 5′–3′ direction. We explored the significance of this gene arrangement. All six possible gene dispositions were examined. In two situations where PR was removed from the leading place and no two genes were in their original location, viral polyprotein processing was abolished. Processing of the polyprotein did not occur when IN was translocated to the front of PR-RT. However, in the following two arrangements, the polyprotein was processed but only at specific sites. First, PR remained in the leading position while the locations of RT and IN were exchanged; viral polyprotein was processed at a site between the upstream transframe peptide (TF) and PR. Second, PR was placed after RT-IN and located at the distal end of Pol. Processing occurred only at the created junction between TF and RT. These results indicated that cleavage after TF occurred autocatalytically but did not proceed to a second site, which needed an extraneous PR for trans-action. Therefore, arranging Pol in the order of PR-RT-IN warrants the streamline processing of the polyprotein once the autocleavage is initiated.