Stepwise introduction of transformable penicillin resistance in Pneumococcus.

ITH the introduction of drug resistances into Pneumococcus by DNA transWf ormation (HOTCHKISS 1951), it became clear that heritable traits not presumably connected to the classic capsular antigens (AVERY, MACLEOD and MCCARTY 1944; AUSTRIAN and MACLEOD 1949) could also be carried by DNA. Certain practical and conceptual developments were derived from this finding. Selective resistance markers allowed quantitative estimation of the number of cells converted, the quantitative improvement of transformation recipes, and the determination of the degree of independence of various marker pairs. Independence of penicillin and streptomycin resistance markers from capsule-type transforming agents was interpreted as indicating the existence of differently marked regions of DNA (considered then to be separate molecules) within the total DNA of a single cell line (HOTCHKISS 1951). The specificity of resistancetransforming units for each kind of drug, and the correspondence of each unit quantitatively to the spontaneous mutational steps of cumulative penicillin resistance, indicated the close parallelism of DNA-carried factors and the units of mutational history present in the cell lines. The original data on penircillin-resistant Pneumococci were briefly reported in 1951 and summarized thereafter at various times. Other demonstrations of transferable drug resistance were soon reported (ALEXANDER and LEIDY 1953) and served, as have later developments in bacterial, and eventually bacteriophage, genetics to demonstrate that DNA can bear a wide variety of traits besides surface antigens. Meanwhile, modifications in the Pneumococcal strains, markers, and techniques used have made it unfeasible to reproduce precisely the conditions of the early experiments. The present article reports a reexamination in the light of modern techniques, of the system which played a considerable part in fashioning our early views on the molecular basis of genetics. In it we describe four identifiable units of penicillin resistance in currently available material, their transfer by DNA to recipient cells, their cooperative phenotypic effects in multiply marked cells, and the indications of certain linkages exhibited by some of them.