Host-dependent modification of bacteriophage T7 and SAMase-negative T3 derivatives affecting their adsorption ability.

When passaging phage T7 and SAMase-negative T3 mutants between E. coli strains with identical (EcoB) or without (EcoO) DNA host specificity, phenotypically a host-controlled modification and restriction is observed. This phenomenon is not due to "classical" modification and restriction of the bacteriophage DNA but depends on the reversibly altered adsorption capacity of the phages on the different host strains.