The fifth nucleobase

Basic ingredient in genetic recipe

Four letter A,T,C,G of genetic alphabet forming three letter words (codon), specifying for 20 amino acids, makes the basic molecular foundation of all life forms. 

But some viruses show a fundamental genomic change, a modified novel fifth base in nucleotide.

Z genome with a noncanonical modified base

In a strategy to escape bacterial restriction enzyme, it was found in the genome of  200 phage virus like one as cyanophage S-2L, out of the four canonical Watson-Crick nucleobases A,T,C,G; A is replaced by the diaminopurine (Z) forming three hydrogen bonds with T, which makes it more stable and rigid from bacterial defenses than double bond A=T. It was first reported in 1977 by soviet researcher. Further study suggest that aminoadenine in place of adenine was included in the phylogeny of these bacteriophage since archaic.

Jordana Cepelewicz in her article further states that: 'Z and other modified DNA bases seem to have evolve to help viruses evade the defenses with which bacteria degrade foreign genetic material. The eternal arms race between bacteriophages and their host cells probably provides enough selection pressure to affect something as seemingly "sacrosanct" as DNA, according to Romesberg.'