278 CELL HEREDITY 



Furthermore, mutant enz\nies can Ix^ characterized h\ the aniouiit of 

 enz\Ti>e activity per cross-Reacting unit 



hi \ctirosfH>ra 25 td mutants ha\e Ixvn studied, all oi them Kx^ahzed 

 in oiH* small rt^on of the genetic map. None contain measurable 

 amounts of the enz\me exci^pt the temperature-sensitixe strain. tii-2A, 

 which has lx>en shown to a^ntain an altered enzvme w hich is more .sen- 

 sitive to hea\\ metal inhibition than is the w ild tvpe. Some of the td 

 mutants form a cross-reacting material iCRM^ which is serologically re- 

 lated to the enzNme. and may represtMit it in an inactive form Some 

 mutants accumulate CRN! and others do not; both classes are re\ertible 

 to w ild t\pe and. therefore, do not represent chromosomal deletions. 

 One of the CRN! -forming mutants, td-2. contains an altered tr\ptophane 

 s\iithetase which can carr\ out reaction 2 but not 1 or 3; thus it re- 

 sembles the .A component of the E. coli enzMiie. 



Some 60 ultra\iolet-induced mutants of E. coli have btvn studied in 

 which tr\ptophane synthetase acti\ity is reduced or absent. Four 

 classes are represented: those lacking .A activity, those lacking B activity, 

 and tho.se w ith altered A or B compt^nents. Tho.se mutants w hich totally 

 lack A or B actixitv do not form an\ CRM. while those with altered 

 activity contain either .A-CRM or B-CRM respectively. These mutants 

 have been mapped by transduction, and the .A and B mutants have been 

 located in separate but closelv linked regions. W ithin each region. 

 CRM -formers and CRM-nonfomiers are not separated from each other 

 but rather thev are interspersed along the map. as shown in Figure 10.3. 



Of the 24 -A mutants which have been studied. 19 are totally itiactiv e. 

 and 5 fonn an altered .A protein w hich is effective in the indole ^ trypto- 

 phane reaction, when combined with normal B. but is inactive in the 

 other two reactions. There are some differences among the five mutants 

 with an altered A component. One of them forms le.ss .A protein and 

 more B protein than the wild tvpe. and another fonns an A protein which 

 is more acid-labile than is the normal A protein. 



.Among the B mutants, four CRM -producers have Ixvn studied which 

 ci^ntain an altered B component. Three of them form a B w hich can 

 only catalv-ze the indolephosphate ^ indole reaction, but is as effective 

 in this reaction as is the nomial B. when a"»nipared in relation to anti- 

 genic specificity. In the fourth strain, the B component exhibits slightly 

 more activity per vmit of antigenic material than dtx's the w ild-tvpe B. 



These results presumably represent just a sampling of the kinds of 

 altered enzvTiies to Ix* found; no attempt has been made to e.xhaust the 

 possibilities. 



These observ ations on the trv ptophane sv^ithetase svstem, as well as 

 the others summarized in Table 10.1, are sufficient to provide evidence 



