Ordering of mutant sites in the isoleucine-valine genes of Escherichia coli by use of merogenotes derived from F 14 : a new procedure for fine-structure mapping.

F-merogenotes derived from F(14) by transductional shortening have previously been found to consist of the sex factor plus one or more of the ilv genes. It is shown here that they carry one or more ilv genes and a variable portion of the adjacent proximal ilv gene. This observation was used to develop a method, analogous to deletion mapping, for ordering mutant sites within the ilv genes. This method requires the use of a series of merogenotes each carrying an increasingly longer segment of the gene being mapped. A simplified method of fine-structure mapping is also described which requires only one or two F' donor strains to map any one gene. This method is based on the large differences observed in recombination frequencies for mutant sites at various distances from the origin of the incomplete merogenote gene. The sequence of 25 mutant sites within three of the ilv genes was determined by use of the simplified procedure.