112 



S. S. COHEN 



This strain of E. coli is unique in having this single metaboHc deficiency. 

 It appears that unless accompanied by other deficiencies which prevent 

 growth, mutation to an unrequited thymine deficiency is lethal. Such 

 mutants would not be recovered in most routine screenmg operations for 

 nutritional mutants. Cell enlargement as a result of DNA deficiency has been 

 observed in certain other nutritionally deficient organisms cultivated in the 

 absence of their requirement. Filament formation has been observed with 

 Thermobacterium acidophilus E,26 cultivated in the absence of deoxyribosides 



20 40 60 80 100 120 

 (min) 



Fig. 20. The irreversible loss of the power to multiply ("thymineless death") as a 

 consequence of withholding thymine from a thymineless strain of E. coli under condi- 

 tions of continuing cytoplasmic growth (Cohen and Earner, 1954). 



(Jeener and Jeener, 1952) and in Lactobacillus leiclimanii or other deficient 

 bacteria cultured in the absence of vitamin Bjo (Deibel et al., 1956; Chaphn 

 and Lochhead, 1956). Filament formation may be induced by a variety of 

 antibiotics, e.g., azaserine, although it is not kno-wTi that the primary effect 

 is on DNA synthesis (Maxwell and Nickel, 1954). However, folic acid analogs, 

 e.g., Amethopterin, also produce this effect, which in some instances is 

 reversed by thymine^ (Nickerson and Webb, 1956). 



^ Indeed, Nickerson and his collaborators have made many contributions to the ex- 

 perimental dissection of growth and division in microorganisms, particularly processes 

 related to the control of dimorphism in the pathogenic fungi. Webb (1953), in turn, has 

 studied similar phenomena in bacteria, arising from magnesium deficiencies of the growth 

 media. 



