Abstract
Heat stable (st) mutants have been isolated from phages T5 (Rubenstein, 1968), T1, T3, T7 (Ritchie & Malcolm, 1970) and λ (Parkinson & Huskey, 1971). In addition to their increased resistance to heat inactivation the st mutant phage particles also have a lower buoyant density and, with the possible exception of T1, contain less DNA than their respective wild-types. All these phages have DNA molecules with non-permuted base sequences (Olligs, 1967; Thomas & MacHattie, 1967) which have been suggested to arise from intracellular concatenated DNA forms (found with T5, T7 and λ) by a sequence specific cutting mechanism which recognizes sites marking the ends of mature DNA molecules (Thomas, Kelly & Rhoades, 1968). Therefore the deletion of non-essential DNA sequences would produce viable phage particles with less DNA than wild-type; this would reduce the buoyant density and apparently also increases the heat stability.