Genetic analysis of two bacterial RNA polymerase mutants that inhibit the growth of bacteriophage T7

The role of the T7 Gp2 inhibitor of host RNA polymerase in phage development

Bacteriophage T7 relies on its own RNA polymerase (RNAp) to transcribe its middle and late genes. Early genes, which include the viral RNAp gene, are transcribed by the host RNAp from three closely spaced strong promoters-A1, A2, and A3. One middle T7 gene product, gp2, is a strong inhibitor of the host RNAp. Gp2 is essential and is required late in infection, during phage DNA packaging. Here, we explore the role of gp2 in controlling host RNAp transcription during T7 infection. We demonstrate that in the absence of gp2, early viral transcripts continue to accumulate throughout the infection. Decreasing transcription from early promoter A3 is sufficient to make gp2 dispensable for phage infection. Gp2 also becomes dispensable when an antiterminating element boxA, located downstream of early promoters, is deleted. The results thus suggest that antiterminated transcription by host RNAp from the A3 promoter is interfering with phage development and that the only essential role for gp2 is to prevent this transcription.

Localization of the Escherichia coli RNA polymerase beta' subunit residue phosphorylated by bacteriophage T7 kinase Gp0.7

During bacteriophage T7 infection, the Escherichia coli RNA polymerase beta' subunit is phosphorylated by the phage-encoded kinase Gp0.7. Here, we used proteolytic degradation and mutational analysis to localize the phosphorylation site to a single amino acid, Thr(1068), in the evolutionarily hypervariable segment of beta'. Using a phosphomimetic substitution of Thr(1068), we show that phosphorylation of beta' leads to increased rho-dependent transcription termination, which may help to switch from host to viral RNA polymerase transcription during phage development.

The genome sequence of Yersinia pestis bacteriophage phiA1122 reveals an intimate history with the coliphage T3 and T7 genomes

The genome sequence of bacteriophage phiA1122 has been determined. phiA1122 grows on almost all isolates of Yersinia pestis and is used by the Centers for Disease Control and Prevention as a diagnostic agent for the causative agent of plague. phiA1122 is very closely related to coliphage T7; the two genomes are colinear, and the genome-wide level of nucleotide identity is about 89%. However, a quarter of the phiA1122 genome, one that includes about half of the morphogenetic and maturation functions, is significantly more closely related to coliphage T3 than to T7. It is proposed that the yersiniophage phiA1122 recombined with a close relative of the Y. enterocolitica phage phiYeO3-12 to yield progeny phages, one of which became the classic T3 coliphage of Demerec and Fano (M. Demerec and U. Fano, Genetics 30:119-136, 1945).

Bacteriophage phiYeO3-12, specific for Yersinia enterocolitica serotype O:3, is related to coliphages T3 and T7

Bacteriophage phiYeO3-12 is a lytic phage of Yersinia enterocolitica serotype O:3. The phage receptor is the lipopolysaccharide O chain of this serotype that consists of the rare sugar 6-deoxy-L-altropyranose. A one-step growth curve of phiYeO3-12 revealed eclipse and latent periods of 15 and 25 min, respectively, with a burst size of about 120 PFU per infected cell. In electron microscopy phiYeO3-12 virions showed pentagonal outlines, indicating their icosahedral nature. The phage capsid was shown to be composed of at least 10 structural proteins, of which a protein of 43 kDa was predominant. N-terminal sequences of three structural proteins were determined, two of them showing strong homology to structural proteins of coliphages T3 and T7. The phage genome was found to consist of a double-stranded DNA molecule of 40 kb without cohesive ends. A physical map of the phage DNA was constructed using five restriction enzymes. The phage infection could be effectively neutralized using serum from a rabbit immunized with whole phiYeO3-12 particles. The antiserum also neutralized T3 infection, although not as efficiently as that of phiYeO3-12. phiYeO3-12 was found to share, in addition to the N-terminal sequence homology, several common features with T3, including morphology and nonsubjectibility to F exclusion. The evidence conclusively indicated that phiYeO3-12 is the first close relative of phage T3 to be described.

Inhibition of Escherichia coli RNA polymerase by bacteriophage T7 gene 2 protein

The 64 amino acid residue product of bacteriophage T7 gene 2 (gp2) binds the Escherichia coli RNA polymerase and inhibits transcription. We localized the gp2 binding site to within 53 amino acid residues in the functionally dispensable region of the RNA polymerase beta' subunit. We investigated the effect of gp2 on transcription at a -10/-35 promoter and at an "extended -10" promoter. Our results indicate that binding of gp2 to the sigma70holoenzyme (Esigma70) prevents promoter recognition at -10/-35 promoters. Once open promoter complexes are formed, however, Esigma70transcription is resistant to gp2, since gp2 can no longer bind RNA polymerase. Surprisingly, transcription inhibition by gp2 is both sigma and promoter-specific. gp2 has little effect on Esigma70transcription from an extended -10 promoter, which does not depend on sigma70region 4 interactions with the -35 promoter box for its activity. gp55-dependent phage T4 late promoter transcription is also resistant to gp2. From these results, we conclude that the interaction of the sigma70region 4 with the -35 consensus promoter element is the primary target of gp2 inhibition.

Bacteriophage T7 DNA packaging. I. Plasmids containing a T7 replication origin and the T7 concatemer junction are packaged into transducing particles during phage infection

Bacteriophage T7 DNA is a linear duplex molecule with a 160 base-pair direct repeat (terminal redundancy) at its ends. During replication, large DNA concatemers are formed, which are multimers of the T7 genome linked head to tail through recombination at the terminal redundancy. We define the sequence that results from this recombination, a mature right end joined to the left end of T7 DNA, as the concatemer junction. To study the processing and packaging of T7 concatemers into phage particles, we have cloned the T7 concatemer junction into a plasmid vector. This plasmid is efficiently (at least 15 particles/infected cell) packaged into transducing particles during a T7 infection. These transducing particles can be separated from T7 phage by sedimentation to equilibrium in CsCl. The packaged plasmid DNA is a linear concatemer of about 40 x 10(3) base-pairs with ends at the expected T7 DNA sequences. Thus, the T7 concatemer junction sequence on the plasmid is recognized for processing and packaging by the phage system. We have identified a T7 DNA replication origin near the right end of the T7 genome that is necessary for efficient plasmid packaging. The origin, which is associated with a T7 RNA polymerase promoter, causes amplification of the plasmid DNA during T7 infection. The amplified plasmid DNA sediments very rapidly and contains large concatemers, which are expected to be good substrates for the packaging reaction. When cloned in pBR322, a sequence containing only the mature right end of T7 DNA is sufficient for efficient packaging. Since this sequence does not contain DNA to the right of the site where a mature T7 right end is formed, it was expected that right ends would not form on this DNA. In fact, with this plasmid the right end does not form at the normal T7 sequence but is instead formed within the vector. Apparently, the T7 packaging system can also recognize a site in pBR322 DNA to produce an end for packaging. This site is not recognized solely by a "headful" mechanism, since there can be considerable variation in the amount of DNA packaged (32 x 10(3) to 42 x 10(3) base-pairs). Furthermore, deletion of this region from the vector DNA prevents packaging of the plasmid. The end that is formed in vector DNA is somewhat heterogeneous. About one-third of the ends are at a unique site (nucleotide 1712 of pBR322), which is followed by the sequence 5'-ATCTGT-3'. This sequence is also found adjacent to the cut made in a T7 DNA concatemer to produce a normal T7 right end.

Protein kinase of bacteriophage T7 induces the phosphorylation of only a small number of proteins in the infected cell

Bacteriophage T7 expresses a serine/threonine-specific, cAMP-independent protein kinase activity encoded by the early gene 0.7. The phosphoproteins specifically resulting from gp0.7 protein kinase expression in T7-infected Escherichia coli have been examined by one-dimensional, SDS-polyacrylamide gel electrophoresis. Only seven major, stable phosphoproteins dependent on gp0.7 protein kinase expression are observed. Two of the gp0.7 protein kinase-specific phosphoproteins observed have been previously identified: the beta' subunit of RNA polymerase and the RNA processing enzyme RNase III. The gp0.7-catalyzed protein phosphorylation activity appears at 9-11 min postinfection at 30 degrees. The new phosphoproteins have a metabolic stability comparable to that of uninfected cell phosphoproteins. T7 protein kinase expression causes the phosphorylation of the same, limited set of proteins in B, C, or K strains of E. coli. Expression of the T3 and BA14 phage protein kinase activities also produces the same phosphoproteins.

Sequence of a conditionally essential region of bacteriophage T3, including the primary origin of DNA replication

The 3526 base-pair nucleotide sequence from near the end of bacteriophage T3 gene 1 to within the coding sequence of gene 2.5 is given. It includes the complete coding sequences for nine known or presumptive proteins, most of which are only conditionally essential for phage growth. The sequence includes five promoters for the phage RNA polymerase, the terminator for early (host enzyme-catalyzed) transcription, and two recognition sites for RNAase III. The primary origin of T3 DNA replication that is utilized by the phage in vivo has been localized to a 142 base-pair region. It has several features in common with the phage T7 origin of DNA replication, and exhibits considerable homology to recognition sites for the mRNA processing enzyme RNAase III. It is proposed that the primary origin of T3 DNA replication may have evolved directly from an RNAase III recognition site. The deletions present in a number of T3 mutant strains and the location of the nucleotide changes in several T3 strains that are defective in their ability to grow on F+-containing strains or on optA mutant hosts have been determined. We discuss how T3 may have become genetically isolated from its relatives in the T7-T3 group and simultaneously acquired novel biological and biochemical properties.

Alterations of the bacteriophage T7 and T3 DNA packaging pathway in Escherichia coli mutant TSN B

Data previously obtained indicate that, during assembly of the related bacteriophages T7 and T3, a DNA-free procapsid (capsid I) is produced and that subsequently capsid I: (1) binds to a longer than mature (concatemeric) DNA and then becomes structurally altered to a particle isolated as a capsid (capsid II) physically resembling the mature bacteriophage capsid more than the procapsid (initiation phase of packaging), (2) draws DNA to its interior (entry phase of packaging), (3) participates in cutting the concatemeric DNA to mature size. It was found that, after infection of Escherichia coli mutant tsnB (selected for a deficiency in plating T7; M. Chamberlin [1974], J. Virol. 14, 509-516), T7 and T3 capsid I is assembled at a rate not significantly different from its rate of assembly in the wild-type host. However, the conversion of capsid I to capsid II was slowed in E. coli tsnB, suggesting that the tsnB mutation interferes with the initiation of DNA packaging. Although some T3 and T7 DNA enters capsids and is cut to mature size in the tsnB mutant, the data further suggest that the entry rate of DNA into capsid II is lower in the tsnB mutant than it is in an unaltered host. T7 capsid II-concatemeric DNA complexes accumulate during infection of the tsnB mutant. These observations suggest that use of the tsnB mutant as a host will simplify studies of bacteriophage T7 and T3 DNA packaging.