CHROMOSOMAL BASIS OF THE MEROZYGOSITY IN A PARTIALLY DIPLOID MUTANT OF PNEUMOCOCCUS

Mary Lee S. Ledbetter ¹ and Rollin D. Hotchkiss ¹

¹ The Rockefeller University, New York, New York 10021

A sulfonamide-resistant mutant of pneumococcus, sul ^r-c, displays a genetic instability, regularly segregating to wild type. DNA extracts of derivatives of the strain possess transforming activities for both the mutant and wild-type alleles, establishing that the strain is a partial diploid. The linkage of sul^r-c to str ^r-61, a stable chromosomal marker, was established, thus defining a chromosomal locus for sul^r-c. DNA isolated from sul^r-c cells transforms two mutant recipient strains at the same low efficiency as it does a wild-type recipient, although the mutant property of these strains makes them capable of integrating classical "low-efficiency" donor markers equally as efficiently as "high efficiency" markers. Hence sul^r-c must have a different basis for its low efficiency than do classical low efficiency point mutations. We suggest that the DNA in the region of the sul^r-c mutation has a structural abnormality which leads both to its frequent segregation during growth and its difficulty in efficiently mediating genetic transformation.