“…In contrast, all classical biotype strains showed hybridization of probe 1 to two PstI or XbaI fragments. Except for strains RJ57 and RJ57(RP4), these results are in agreement with published reports and confirm that the CT operon is present at two locations in classical strains and at one location on the chromosome of most El Tor strains (12,14,16,19,24,25). The third PstI and XbaI fragments observed in our experiments with DNA from strains RJ57 and RJ57(RP4), which are derived from El Tor strain RV79 (10), presumably hybridized with the flanking sequences of probe 1 and not with CT operon-specific sequences (16).…”