Defective transcription of the right end of bacteriophage T7 DNA during an abortive infection of F plasmid-containing Escherichia coli

F exclusion of bacteriophage T7 occurs at the cell membrane

The F plasmid PifA protein, known to be the cause of F exclusion of bacteriophage T7, is shown to be a membrane-associated protein. No transmembrane domains of PifA were located. In contrast, T7 gp1.2 and gp10, the two phage proteins that trigger phage exclusion, are both soluble cytoplasmic proteins. The Escherichia coli FxsA protein, which, at higher concentrations than found in wild-type cells, protects T7 from exclusion, is shown to interact with PifA. FxsA is a polytopic membrane protein with four transmembrane segments and a long cytoplasmic C-terminal tail. This tail is not important in alleviating F exclusion and can be deleted; in contrast, the fourth transmembrane segment of FxsA is critical in allowing wild-type T7 to grow in the presence of F PifA. These data suggest that the primary event that triggers the exclusion process occurs at the cytoplasmic membrane and that FxsA sequesters PifA so that membrane damage is minimized.

Isolation and identification of fxsA, an Escherichia coli gene that can suppress F exclusion of bacteriophage T7

A selection for mutants of Escherichia coli that survive coexpression of bacteriophage T7 gene 10 and plasmid F pifA has allowed the identification of a newly defined genetic locus, fxsA. fxsA is located at 94.1 min on the E. coli chromosome; the gene is monocistronic and non-essential for growth. Overexpression of fxsA is necessary for resistance to the toxicity of T7 gene 10 in the presence of pifA; the original mutant strain contains a promoter-up mutation, changing a G residue to the "invariant" T in the -10 hexamer of a sigma(70)promoter. This chromosomal mutation causes a 25-fold increase in the level of fxsA mRNA. The initiation codon of fxsA is shown to be UUG, and the FxsA protein is then deduced to consist of 158 amino acid residues. A similar mutant selection that demanded cell survival to a challenge of T7 gene 1.2 and pifA also resulted in the isolation of the identical promoter-up mutation that affects expression of fxsA. The increased levels of FxsA resulting from the promoter-up mutation allow phage T7 to avoid exclusion by the F plasmid, presumably by protecting the cell from premature death due to gene 10 or to gene 1.2 expression in the presence of the PifA protein.

Expression, regulation, and mode of action of the AbiG abortive infection system of lactococcus lactis subsp. cremoris UC653

The abortive infection system AbiG is encoded by the lactococcal plasmid pCI750. The abiG locus (consisting of two genes, abiGi and abiGii) was examined by Northern blot analysis, revealing two transcripts of approximately 2.8 and 1.5 kb which were homologous to the two gene-specific probes. A transcriptional start site was mapped upstream of abiGi, and it appeared that the two genes were cotranscribed, resulting in the 2.8-kb transcript. The smaller transcript may be the result of independent transcription of abiGii within abiGi or of the presence of a weak terminator within abiGii. The locus was shown to be constitutively expressed. Evidence is presented for the possible existence of a second Abi mechanism on pCI750. Examination of phage sk1 RNA synthesis demonstrated that both the subcloned AbiG and, to a greater extent, pCI750 inhibited this process. pCI750 also severely inhibited synthesis of both early and late phage c2 transcripts, while the presence of the subclone resulted in a reduction in late transcript synthesis only.

Rate of translocation of bacteriophage T7 DNA across the membranes of Escherichia coli

Translocation of bacteriophage T7 DNA from the capsid into the cell has been assayed by measuring the time after infection that each GATC site on the phage genome is methylated by cells containing high levels of DNA adenine methylase. Methylation at GATC sites on T7 DNA renders both the infecting genome and any newly synthesized molecules sensitive to the restriction enzyme DpnI. In a normal infection at 30 degrees C, translocation of the T7 genome into the cell takes between 9 and 12 min. In contrast, translocation of the entire phage lambda genome or of a T7 genome ejected from a lambda capsid can be detected within the first minute of infection. Entry of the leading end of the T7 genome occurs by a transcription-independent mechanism that brings both Escherichia coli and T7 promoters into the cell. Further translocation of the genome normally involves transcription by the RNA polymerases of both E. coli and T7; the rates of DNA translocation into the cell when catalyzed by each enzyme are comparable to the estimated rates of transcription of the respective enzymes. A GATC site located between the early E. coli promoters and the coding sequences of the first T7 protein made after infection is not methylated before the protein is synthesized, a result supporting the idea (B. A. Moffatt and F. W. Studier, J. Bacteriol. 170:2095-2105, 1988) that only certain proteins are permitted access to the entering T7 DNA. In the absence of transcription, the genomes of most T7 strains do not completely enter the cell. However, the entire genome of a mutant that lacks bp 3936 to 808 of T7 DNA enters the cell in a transcription-independent process at an average overall rate of 50 bp per s.

Incomplete entry of bacteriophage T7 DNA into F plasmid-containing Escherichia coli

The penetration of bacteriophage T7 DNA into F plasmid-containing Escherichia coli cells was determined by measuring Dam methylation of the entering genome. T7 strains that cannot productively infect F-containing cells fail to completely translocate their DNA into the cell before the infection aborts. The entry of the first 44% of the genome occurs normally in an F-containing cell, but the entry of the remainder is aberrant. Bypassing the normal mode of entry of the T7 genome by transfecting naked DNA into competent cells fails to suppress F exclusion of phage development. However, overexpression of various nontoxic T7 1.2 alleles from a high-copy-number plasmid or expression of T3 1.2 from a T7 genome allows phage growth in the presence of F.

Genes 1.2 and 10 of bacteriophages T3 and T7 determine the permeability lesions observed in infected cells of Escherichia coli expressing the F plasmid gene pifA

Infections of F plasmid-containing strains of Escherichia coli by bacteriophage T7 result in membrane damage that allows nucleotides to exude from the infected cell into the culture medium. Only pifA of the F pif operon is necessary for "leakiness" of the T7-infected cell. Expression of either T7 gene 1.2 or gene 10 is sufficient to cause leakiness, since infections by phage containing null mutations in both of these genes do not result in permeability changes of the F-containing cell. Even in the absence of phage infection, expression from plasmids of either gene 1.2 or 10 can cause permeability changes, particularly of F plasmid-containing cells. In contrast, gene 1.2 of the related bacteriophage T3 prevents leakiness of the infected cell. In the absence of T3 gene 1.2 function, expression of gene 10 causes membrane damage that allows nucleotides to leak from the cell. Genes 1.2 and 10 of both T3 and T7 are the two genes involved in determining resistance or sensitivity to F exclusion; F exclusion and leakiness of the phage-infected cell are therefore closely related phenomena. However, since leakiness of the infected cell does not necessarily result in phage exclusion, it cannot be used as a predictor of an abortive infection.

Expression of gene 1.2 and gene 10 of bacteriophage T7 is lethal to F plasmid-containing Escherichia coli

Plasmids expressing bacteriophage T7 gene 1.2 or gene 10 DNA transform F plasmid-containing strains of Escherichia coli only at low efficiency, though they transform plasmid-free strains normally. The gene products T7 gp1.2 and T7 gp10 appear to be the toxic agents, and their effects are directed towards the product of the F pifA gene, PifA. T7 gp1.2 and gp10 are also the two targets of the pif exclusion system of F, and their synthesis normally triggers the abortive infection of T7 in pifA+ hosts. The properties of plasmids containing T7 gene 1.2 or 10 suggest that they can be used to study the molecular mechanisms of phage exclusion in model systems that avoid the pleiotropic dysfunctions associated with an abortive infection.