Transforming principle identified by Avery was
Oswald Avery, Colin MacLeod, and Maclyn McCarty in 1944 purified transforming principle from heat-killed S pneumococci. They sequentially removed lipids, polysaccharides, and proteins with enzymes, showing activity remained after protease and RNase treatments but was abolished by deoxyribonuclease. Chemical analysis revealed DNA with high viscosity, phosphorus content, and diphenylamine reaction positive. Transformation of rough to smooth required intact DNA, proving DNA rather than protein carries genetic specificity. This milestone established DNA as genetic material preceding Hershey-Chase
Ref: Avery et al., Journal of Experimental Medicine 1944: Studies on Chemical Nature of Transforming Principle; Hartl, Chapter 1