A study of Pneumococcal merodiploids at the molecular level.

The DNA of a sulfonamide-resistant Pneumococcal strain (heterozygous for sulr-c) and that of three highly resistant and persistently heterozygous cd transformants, derived by introducing sulr-c marker into a stable sulfonamide resistant strain (sulr-d), were studied to analyze the genetic basis of their merodiploidy. The physical properties of the native and denatured DNA from the heterozygotes and the nonheterozygous strains were not distinguishable. The denaturability and the renaturability of biological activity for the heterozygous markers were essentially identical to those of the normal markers. The heterozygosity extends to the closely linked locus giving rise to four different configurations of cd and cd+ transformants, characterized by their frequencies of segregation and donor-marker activities. The marker-activity ratios and the frequency of co-transfer of heterozygous markers were found to remain the same in each when the donor DNA was native, denatured or reannealed without fractionation or reannealed after remixing of resolved strands. Possible models were weighed against these observations and these considerations led to the suggestion that tandem duplication of a gene region may be responsible for the heterozygosity and instability of this region. A more detailed examination of this model will be presented in an accompanying paper.