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The Inability of T-even Phages to Produce Heat-stable Density Mutants and its Bearing on Chromosome Maturation

Institute of Virology University of Glasgow Glasgow G11 5JR Scotland

Heat stable (st) mutants have been isolated from phages T5 (Rubenstein, 1968), T1, T3, T7 (Ritchie & Malcolm, 1970) and (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 ) 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.

Received 24 January 1972; accepted 30 March 1972.