452 



CHAPTER 35 



cosyl transferases. These enzymes transfer 



glucose from uridine diphosphate glucose 

 (UPP glucose) — not shown in Figure 35- 

 1 — to HMC residues in DNA. Such en- 

 zymes are not found in uninfected or T5- 

 infected cells and are clearly phage-induced. 

 Note again that the glucosyl transferases act 

 on polydeoxyribotid.es. 



We have seen, therefore, that after T- 

 even phage infection, new enzymes are in- 

 duced to carry out syntheses unique to viral 

 DNA production and to neutralize host en- 

 zymes which would be antagonistic to this 

 process. New enzymes are also known to 

 supplement the action of the host's enzymes 

 to speed up synthesis of viral DNA. (For 

 example, phages induce production of a dif- 

 ferent thymidilate synthetase than their 

 host's. ) These results not only indicate that 

 viral DNA leads to the destruction of the 

 host's DNA, but they provide us with some 

 insight as to where DNA synthesis is regu- 

 lated genetically in the T-even phage-/:, coli 

 system. 



Variation in Genetic Nucleic Acid Components 



The base-ratio of double-stranded DNA 

 containing A, T, C, and G can be estimated 

 from its buoyant density in the ultracentri- 

 fuge and from its denaturation (melting) 

 temperature. A discrepancy in the base- 

 ratios found by these methods :t for the 

 DNA of phage PBS 1 (and also PBS 2) 

 is explained by the finding that all the T 

 in the phage DNA is replaced by U, the 

 base composition frequencies being A = 

 0.359, U = 0.359, G = 0.134, and C = 

 0.147. The host of this phage, Bacillus 

 subtilis, has T not U in its DNA. Appar- 

 ently information which the phage carries 

 in its own genome incorporates </UP into 

 its DNA to the exclusion of dTP. 



PBS 1 can transduce several of the ge- 

 netic markers of its host. Transducing 



-See I. Takahashi and J. Marmur (1963). 



phages like P22 and A have a DNA base 

 composition similar to their host's DNA. In 

 the present case, the G -f- C content of host 

 and phage are quite different — 43% and 

 28%, respectively. Consequently, the PBS 

 1 (PBS 2) -Bacillus system seems to offer an 

 unusual opportunity to study the mechan- 

 ism of transduction as well as the genetics, 

 biosynthesis, and homology of host and 

 phage DNA. 



HMC (with or without attached glucose) 

 and U are not the only genetically-deter- 

 mined variations which occur in the bases 

 incorporated into DNA. (Various bases 

 found in native DNA have already been 

 mentioned on pp. 254-255.) Still other py- 

 rimidines appear in DNA. 5-Methyl cyto- 

 sine occurs in higher organisms like wheat, 

 mammals, fish, and insects, with higher per- 

 centages of this base found in plants than 

 in animals. On the other hand, 5-methyl 

 cytosine is absent from many microorgan- 

 isms — bacteria, actinomycetes, yeasts and 

 their relatives, algae, and protozoa. Trace 

 amounts of 5-ribosyl uracil are reported to 

 occur in DNA. Finally, thymine is prob- 

 ably replaced by 5-bromo uracil in infec- 

 tious bovine rhinotracheitis virus 4 and by 

 5-hydroxymethyl uracil in a phage/' 



DNA can contain a variety of purines. 

 Although 5-methylaminopurine (6-methyl 

 adenine) is not found in the DNA of acti- 

 nomycetes, yeast, higher plants, or higher 

 animals, it is found in some bacteria — for 

 example, E. coli, Aerobacter aerogenes, Dip- 

 lococcus pneumoniae, and Mycobacterium 

 tuberculosis bovis — and related bacterio- 

 phages. Not more than 0.7% of all bases 

 is made up of 5-methylaminopurine. Trace 

 amounts of 2-methylamino guanine, 6-di- 

 methylaminopurine, 1 -methyl guanine, and 

 2-methyl adenine are reported to occur in 

 DNA. Unlike the findings for pyrimidines. 



* See J. G. Stevens and N. B. Groman ( 1963). 

 r ' See D. H. Roscoe and R. G. Tucker ( 1964). 



