MUTAGENESIS 139 



Using recombination frequency as a measure of distance, one can 

 try to correlate the distance between two mutations measured in the 

 recombination units with the number of intervening sites. As far as 

 this goes, there is no apparent large chunk of the structure which is 

 quiet. 



In the map shown (Fig. 22), segments are drawn proportional to the 

 number of sites known in that particular segment. These segments are 

 arbitrary, being determined by the end points of the various deletions 

 used to map or define the various segments. But they define the 

 sequence of the respective portions of the map. 



The figure shows a series of mutants that were mapped by Chase 

 and Doermann (10), without reference to the deletions at all, just by 

 recombination frequency. They are spaced according to the recombina- 

 tion frequency. The first thing to see is that the sequence correlates 

 precisely, without exception, to the sequence determined by segment 

 mapping. Down below is another group mapped by Edgar, Feynman, 

 Klein, Lielausis, and Steinberg, also without reference to deletions. 

 Again, there is a perfect correlation as far as sequence is concerned. 



From these data, one can make a judgment as to the existence of 

 quiet regions. If they exist, they would show themselves by discrepan- 

 cies in spacing of mutations in the two kinds of maps. If there are any 

 they obviously don't involve any enormous part of the structure. 

 Certainly, at the level of fine detail, there is a lot of very close 

 clustering of neighboring sites, but that is something else again. 



I mention this because the question of quiet regions came up in Dr. 

 Demerec's talk, and there have been reports of large quiet regions in 

 Neurospora, also. This doesn't seem to be so in the case of r II 

 mutants. It is important to point out that in this system the function 

 under study is an expendable one, so that any mutation leading to 

 loss of the function can be detected. In some other systems only a 

 special class can be seen, for instance, those in which one functional 

 protein is altered to a different (but also functional) form. In such a 

 case, one would expect quiet regions. 



Getting back to mutation rates, what you measure as the rate of 

 production of r II mutations is a composite of at least 400 different 

 individual sites that are contributing to this rate. The way to measure 

 mutation rates in r II mutants is simply to put into a tube some bac- 

 teria plus a few phage particles of the standard type, so that the 

 chance of introducing a mutant is negligible, let them grow up into a 

 lysate, and then measure the fraction of particles that are mutant. 



The proportion of mutant particles actually measured is the muta- 

 tion rate times the number of generations, which gives a number about 



