Synthesis of Bacteriophage-Coded Gene Products During Infection of Escherichia coli with Amber Mutants of T3 and T7 Defective in Gene 1
Journal of Virology
During nonipermissive infectioin by a T7 amber mutant in gene 1 (phage RNA polymerase-deficient), synthesis of the products of the phage genes 3 (endonuclease), 3,5 (lysozyme), 5 (DNA polymerase), and 17 (serum blocking power) was shown to occur at about half the rate as during wild-type infection. This relatively high rate of expression of "late" genes (transcribed normally by the phage RNA polymerase) seems to be a general feature of all T7 mutants in gene 1 from our collection. In contrast,
... tion. In contrast, T3 gene 1 mutants and a T7 gene 1 mutant from another collection showed late protein synthesis at very reduced rates. Synthesis of the gene 3 endonuclease by T7 gene 1 mutants was very sensitive to the addition of rifampin 2 min after infection, conditions under which there was very little inhibition during wild-type infection. This supports the notion that late gene expression during nonpermissive infection by gene 1 mutants is dependent on the transcription of the T7 genome by the host RNA polymerase. In contrast to T3 gene 1 mutants, the T7 gene 1 mutants of our collection directed the synthesis of phage DNA during nonpermissive infection. This DNA accumulated as a material sedimenting faster than mature T7 DNA. The RNA polymerases (gene 1 products) coded by the related phages T3 and T7 have a central role regarding the pattern of gene expression of these viruses: the RNA polymerase of the host, Escherichia coli, is only able to efficiently transcribe a short segment at the "left" end of the phage genomes. In the case of T7 this segment has been shown to code for at least four proteins ("early" proteins), one of which is the phage DNA-specific RNA polymerase. The rest of the approximately 30 genes coded by these phages is efficieintly transcribed by the phage RNA polymerase only. Thus, it was postulated that a terminator, i.e., a nucleotide sequence leading to the interruption of transcription by the host RNA polymerase, was located near the "right" end of the ligase gene (see reviews by Studier  and Summers [Annu. Rev. Genet. 6, in press]). Davis and Hyman (3) have found visual evidence for such a terminator, at about 20% of the length of the T7 genome, through electron microscopy of T7 DNA transcribed in vitro. And Studier (20) has located this terminator to the right of gene 1.3 (ligase). In addition to this terminator, tl, another terminator, t2, has been postulated to occur at about 30% of the genome, coming from the left (Summers, Annu. Rev.