T-even phages

Fig. 3.3 T-even phage stracture before and after tail contraction.

A remarkable example of self-assembly is that of the T-even phage (Box 7-C).213 215 From genetic analysis (Chapter 26) at least 22 genes are known to be required for formation of the heads, 21 genes for the tails, and 7 genes for the tail fibers. Many of these genes encode... 

Bacteriophage, a virus infecting bacterial cells, has a structure somewhat different from those previously described. A head contains the nucleic acid and the viral DNA passes through a tail during the infection process. In the T-even phages (Fig. 1), the tad consists of a tube surrounded by a sheath and is connected to a thin collar al the head end and a plate at the tip end. The sheath is capable of contraction and the plate possesses pins and tail fibers, which are the organs of attachment of the bacteriophage to the. wall of the host cell. 

Becker and Hurwitz 94) have found that after infection of E. coli B with T-even bacteriophages a novel 3 -deoxynucleotidase activity appears. They purified the enzyme 2000-fold. In addition to its attack on 3 -deoxymononucleotides, the enzyme selectively removes the 3 -phos-phoryl groups from DNA. It does not attack 3 -ribonucleotides, 3 -phosphoryl groups of RNA, or 5 -phosphate esters. Like bacterial 5 -nucleotidases, this enzyme is markedly activated by Mg2+ and Co2+ and is inhibited by EDTA. The enzyme appears to be a phage-induced enzyme the activity rises early after injection with T-even phages and formation of the enzyme is blocked with chloramphenicol. 

Deoxyribonucleic acid is the only nucleic acid found in T-even phage,88 and earlier reports89 of the presence of ribonucleic acid in bacterial virus T2 and T6 in small, variable proportions are now attributed88 to a contamina-... 

Figure 9-27. Buoyant densities (25°C) of native DNA s in CS2SO4 (A) and CsCl (B) gradients as a function of base composition ( , solid lines), expressed as mole percent of G + C or G + HMC, or as a function of the glucose to HMC ratio (mole %), the latter for T-even phage DNA (O, dashed lines). Denatured E. coli DNA (Den. E COLI, 0) was prepared by heating DNA (10 jag/ml) for 10 minutes at 96 to 100°C in 0.02M sodium citrate (pH = 7.8) and rapidiy chilling to 0°C. Based largely on the data of R. L. Erikson and W. Szybalski, Virology, 22 111 (1964). [From W. Szybalski, in Methods of Enzymology, Vol. 12B, Academic Press, New York, 1968, p. 330.]...

Immunization with denatured DNA-MBSA gives rise to antibodies, often mainly of the IgM class, that react with single-stranded DNA but not with native DNA. The largest antigenic determinant for such antibodies is about the size of a pentanucleotide. When DNA with an unusual base, such as the glucosylated hydroxymethylcytosine of T-even phage, is used, the specificity is directed largely to the modified base and much more IgG may be produced.Similarly, when UV-irradiated DNA or photooxidized DNA is used, the modified bases of the lesions provide the major specificity determinants. - ... 

All are excised by a similar mechanism. Many are selfsplicing, but others require a protein catalyst. A similar splicing sequence is involved in removal of a 1017-nucleotide intron from the thymidylate kinase gene of phage T4 and other introns in T-even phage. The later are among the relatively rare introns in prokaryotic systems... 

T], T2, T, T7 and X have similar structures to those of the T-even phages, but the tail is less rod-like (i.e. it is more flexuous), and it does not contract during the infection process. 

Biosynthesis of S-hydroxymethyldeoxycytidylic acid. This component of the DNA of T-even phages is biosynthesized from deoxycytidine 5 -monophos-phate (Fig. 3). Pyrimidine nucleotides may aim be product by the Salvage pathway (see). 

Terminal redundancy the existence of identical genetic information at each end of a viral chromosome. In X-phages this constitutes up to 20 nucleotide pairs, whereas up to 6,000 nucleotide pairs may be terminally redundant in T-even phages. 

Modification of tRNA following T-Even Phage Infection OF E. coli... 

Further studies were concentrated on the factors leading to the appearance of Leu-tRNA p, accompanied by the decrease in the level of Leu-tRNAj. This alteration, which requires both phage DNA and phage protein synthesis, is apparent after infection by all T-even phages but not following infection with T-odd phages or after induction of phage A. 

The chart in Fig. 2 shows an alternate path for the formation of dUMP by direct deamination of dCMP. This may be how cytidine could be converted to thymidylate in the cases cited above [125,126]. However, this deaminase is not usually detected in E. coli but is induced by infection with T(even) phages [132,133]. It has also been purified from chick embryo and mammalian tissues, and its properties have been extensively analyzed [134-136]. It acts as a typical allosteric enzyme in both the phage-infected E. coli and animal systems. Homotropic substrate interaction is evident, and this is modified by dCTP as an activator, and by dTTP (sometimes dGMP) as an allosteric inhibitor. This type of control apparently functions to regulate the level of dTTP by feedback inhibition and by activation when the supply of dTTP is depleted. Cytidine deaminase (EC 3.5.4.5) isolated from sheep liver [137] appears to have the same allosteric properties, with the same positive and negative effectors, as those of dCMP deaminase. The latter enzyme is also induced by phage infection in B. subtiUs, and in contrast to the deaminase from all other sources it does not show allosteric inhibition or activation by any nucleotide [138]. 

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