ON THE TOPOGRAPHY OF THE GENETIC FINE STRUCTURE

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Abstract
A small portion of the genetic map of phage T4, the two cistrons of the rII region, has been dissected by overlapping "deletions" into 47 segments. If any branch exists, it cannot be larger than one of these segments. The overlapping deletions are used to map point mutations and the map order established by this method is consistent with the order established by the conventional method that makes use of recombination frequencies. Further dissection has led to the identification of 308 distinct sites of widely varied spontaneous and induced mutability. The distributions throughout the region for spontaneous mutations and those induced by various chemical mutagens are compared. Data are included for nitrous acid and ethyl methane sulfonate acting in vitro, and 2-aminopurine, 2,6-diaminopurine, 5-bromouracil, 5-bromodeoxycytidine, and proflavine acting in vivo. The characteristic hotspots reveal a striking topography.