RECOMBINATION ANALYSIS IN MICROBIAL SYSTEMS 61 



other mutant. Where mutants are non-identical and not overlapping, 

 maltose-positive recombinants are formed. Table I shows the results of 

 these crosses, from which an approximate arrangement of the mutants 

 can be made by rapid inspection. Mutant e gives no recombination to 

 wild type with any other mutant, while mutant d gives recombinations 

 only with c and h. On the other hand, mutant i gives recombination 

 witli mutants g, /, and / but not with the others. It may be supposed 

 that mutants d, e, and i are either deficiencies or extended mutant 

 sequences, carrying the segments occupied by the mutants with which 

 they show no recombination. One obtains, thus, a first general arrange- 

 ment of the mutated sites. 



ch fgi 



An idea of the order of c, h, g, f, and / is obtained if one examines the 

 wild-type frequencies yielded when a given recipient strain is treated 

 by DNA of each of the other mutants. In all instances the order appears 

 to be c, h, g, /, and /. 



Closer inspection of the table reveals, however, some striking fea- 

 tures. Most of the crosses do not give the same result in reciprocal ar- 

 rangements. For example, when mutant c is the recipient and / the 

 donor, one observes the production of 0.34 per cent wild type by re- 

 combination. However, when / is recipient and c the donor, the per 

 cent wild type is only 0.12. This difference is too great, and the inci- 

 dence of this kind of result too frequent, to be devoid of significance. 

 What will be done now is to consider this feature of recombination 

 in transformation as an intrinsic property of the mechanism by which 

 recombination takes place, and we shall let it dictate a model for re- 

 combination at the molecular level. 



A model of recombination in transformation 



The first task is to try to discern a general principle related to 

 these asymmetries in reciprocal recombination frequencies. Mutants 

 d and i, which give wild-type recombinants with certain other mutants, 

 but not all, clearly involve large segments of the locus; d covers at least 

 three mutational sites, and i covers at least two. In crosses involving 

 these mutants, very low frequencies of wild recombinants are encoun- 

 tered whenever the "large" mutant, d or i, is in the recipient cell. One 



