186 ROLLIN D. HOTCHKISS AND AuDREY H. EvANS 



transformants like the original highly resistant strain Fn, now 

 therefore considered to be Fadb or Fbad. Each of the pair 

 combinations as DNA donors in transformation gave rise to 

 the expected transformants of their own phenotype and also 

 of the unit types of which they are "composed". 



Determining quantitatively the proportions of the various 

 transformants when Fn is the donor, it has been inferred that 

 its units have the probable sequence adb, in which the pair 

 ad is linked closely and the pair db is loosely linked. The 

 linkage between a and b seems to be lower and is an indication 

 that the donor Fn with a similar linkage has the genetic 

 composition adb. All seven possible type of recombinants 

 have been made by transformation of the sensitive strain, 

 and the relative frequencies of their production are in accord- 

 ance with this arrangement. It is probable that frequencies of 

 linkage in such minute recombinations are determined not 

 only by distances but also by preferential breakage points, 

 and limitations upon the size of the fragment of DNA can be 

 incorporated. 



Here, then, is a strain, Fn, whose high resistance to sul- 

 phonamides is determined by its genie substance; in parti- 

 cular, its DNA. This DNA is passed on intact from mother 

 to daughter cells at division, but on incorporation into new 

 cells in transformation is usually fragmented. The three 

 regions {a, d, b) of alteration in the mutant DNA represent 

 another example of genetic fine structure (see Demerec, 

 Blomstrand and Demerec, 1955; Benzer, 1955) which seems 

 to be within a single gene. Of interest in the present case is 

 the fact that this linkage of subunits is displayed by chemic- 

 ally purified DNA. Furthermore, it is a welcome feature in 

 this instance that each of the genotypes a, d, b, ad, etc. 

 obtained by disseminative or dispersive transformation has 

 a distinctive phenotype providing its own basis for quantita- 

 tive recovery. In previously investigated cases, the recom- 

 bining variants all have the same phenotype, a certain bio- 

 chemical or physiological lack, and cannot be distinguished 

 from each other. 



