230 ERNST FREESE 



1)V liydiM/.iiic air coiiipai-ahlc to oi' lowci' tliaii those of tl-catiiicnt hy low 

 |)11 oi- hy cthyhitin^ agents (Fivese ct (il., HMilh). This ohsei'vation 

 a-j;ain iiKhcatcs that the luutagenic effect of HA is predominantly caused 

 hy its I'caction with C. 



3. Exposure to Low pll 



When one exposes DNA to solutions l)eIow pH 4 some bases (whose 

 pK value for the amino groups is in this pH range) become positively- 

 charged and strand separation occurs (Marmur et al, 1961) which may 

 be lethal. At or above pH 4 this effect is negligible, but low pH can affect 

 DNA by two additional reactions wliich sliould be active even above 

 pH 4. 



a. Dcpurhiation. The complete removal of A and G by low ])H has 

 been reported by Tamm et al. (1952). Although these measurements have 

 only been made in the pH ranges 1.6 to 4 it is clear that some depuri- 

 nating effect must remain at higher pH, especially at elevated tempera- 

 tures. At pH 1.6 the authors observed that G is liberated initially more 

 readily than A. This should be measured again for pH ranges near 4.2, 

 at which the mutagenic experiments are usually done. 



No other effect of low pH on the nucleic acid bases has been reported, 

 and extensive hydrolysis by low pH has often been used to isolate and 

 quantitate the DNA bases. 



b. Backbone Breakage. The sugar-phosphate backbone of DNA could 

 be labile to treatment at low pH. This must be a minor effect since 

 extensive treatment at pH 1.6 by the above authors gave "apurinic acid" 

 without too much backbone breakage. However, when this apurinic acid 

 is exposed to solutions at high pH all sugar-phosphate bonds which have 

 no base attached to the sugar, apparently break, for the phosphate 

 appears in the dialyzate (Tamm et al, 1953). Even at neutral pH this 

 breakage occurs, although very slowly. It is apparently caused by 

 "/3-elimination" (Bayley et al, 1961). Since the biological material 

 after exposure to low pH is returned to neutral pH some of these breaks 

 may occur. 



Exposure of T4 phages to pH 4.2 induces mutations most of which 

 arc point mutations (Fi'ecsc, 1959('l. It seems unlikely that such a small 

 change can be induced by a break in the DNA backbone; a break should 

 rather induce larger alterations or, more frequently, be lethal to the 

 phage. Hence it is probable that the observed point mutations have been 

 induced by depurination without any subsequent backbone breakage. 



The gap left by the purine removal might be filled again before the 

 DNA replicates. No such mechanism is known so far. Alternatively, the 

 gap could remain empty until the DNA replicates. DNA replication 



