Functional Grouping of Genes in Phages T4 and T7
The bacteriophage T4 has a large genome in which there are (i) essential genes and (ii) nonessential genes. These genes exhibit extensive functional grouping, but we do not fully understand the significance of the presence of so many non-essential genes. There are three phases of gene expression : (i) early genes, (ii) quasi-late genes and (iii) late genes. The early genes and quasi-late genes are concerned with the synthesis of DNA, modifying cell structure or with transcribing and translating phage genes. Two of these genes involved in transcription are necessary for late gene
expression. However, we do not know, how the cascade of three phases of gene expression is controlled.
Like all other phages, T7 also has three classes of genes called class I, class II and class III genes. The enzymes synthesized on phage genes interfere with host gene expression and the synthesis of phage RNA polymerase. The phage enzymes synthesized by T7 genes are responsible for expression of class II genes and class III genes. The basis for the difference in expression between class II and class III genes is not known.