120 CHARLES A. THOMAS, JR. 



(leinonstrate that ^X piiage particles <;tain bright red with acriflinc 

 orange, wliile T2 phage stain yellow-green (Anderson et al., 1959). 



A vciy sensitive method for the detection of single-ehain regions has 

 been perfected l)y Levine, Murakami, Van Vunakis, and Grossman 

 (1960), based on the complement fixation that attends the reaction of an 

 antibody to DNA which is specific to denatured DNA. 



In conclusion, then, one can say that the molecules liberaterl from 

 T2 and T4 have very little or no single-chain regions while the majority 

 of ^X DNA is single-chained. Molecules of intermediate type have not 

 yet been identified. At jiresent it is not known whether some hydrogen 

 bonds are broken during the folding of the DNA into its compact form. 



5. Speculations on DNA Organization 



If the DN^A is uniquely folded into a compact structure prior to the 

 assembly of the head membrane about it, as is suggested by the work 

 of Kellenberger, it is reasonable to suspect that at least pavi of the 

 ability to form specific folds is due to some structural features along 

 the DNA molecule. The presence of such regions along the DNA mole- 

 cule which are concerned only with the determination of the tertiary 

 folding have been suggested by ]\Iahler and Fraser (1961). From the 

 chemical point of view, there is the unexplained observations of Dunn 

 and Smith (1959) who find about 1 mole of 6-methylaminopurine for 

 eveiy 200 moles of adenine in T2 phage. This corresponds to about 

 500-600 of such strange bases per phage particle. Since such residues 

 could not form the normal hydrogen bonds with thymine, these bases, 

 if present, might cause an alteration in the nonnal duplex structure of 

 the molecule. It is interesting to note that the average number of 

 internal protein molecules per phage is about 300 calculated on the basis 

 of the amount of this protein, about 5% (Levine et al., 1958; Minagawa, 

 1961), and a molecular weight of 15,000 which is based on methionine 

 content (Rubenstcin, 1960). In order to compact the DNA into the small 

 volume of the phage head about 300 to 600 turns or folds must be made. 

 The coincidence in the magnitude of these numbers leads one to suspect 

 that they result from some special requirements of the organization of 

 the DNA in the phage particle. 



In experiments on the reassembly of protein subunits around TMV- 

 RNA, it seems clear that the RNA plays an important role in the 

 reassembly process. First, because the reconstitution process can occur 

 at higher pH values than wnll permit the reassembly of the protein units 

 alone, and second, because the length of the TMV rod assumes a unique 

 value — presumably corresponding to the length of the coiled RNA 

 molecule (Fraenkcl-Conrat, 1962; Fraenkel-Conrat and Singer, 1959; 



