A certain gene in one strain of E. coli can be cleaved into fragments of 3 kb and 4 kb using the restriction endonuclease PstI, but it is unaffected by PvuII. In another strain of bacteria, the same gene cannot be cleaved with PstI, but treatment with PvuII yields fragments of 3 kb and 4 kb. Explain the probable genetic difference between the two strains.