Abortive infection of Escherichia coli F+ cells by bacteriophage T7 requires ribosomal misreading

Cloning and DNA sequence analysis of two abortive infection phage resistance determinants from the lactococcal plasmid pNP40

The lactococcal plasmid pNP40, from Lactococcus lactis subsp. lactis biovar diacetylactis DRC3, confers complete resistance to the prolate-headed phage phi c2 and the small isometric-headed phage phi 712 in L. lactis subsp. lactis MG1614. A 6.0-kb NcoI fragment of pNP40 cloned in the lactococcal Escherichia coli shuttle vector pAM401 was found to confer partial resistance to phi 712. Subcloning and deletion analysis of the recombinant plasmid pPG01 defined a 2.5-kb ScaIHpaI fragment as conferring phage insensitivity. Sequence analysis of this region confirmed the presence of two overlapping open reading frames (ORFs). Further subcloning of pNP40 to characterize the resistance determinant active against phi c2 identified a 5.6-kb EcoRV fragment of pNP40 which, when cloned in pAM401, conferred partial resistance to both phi c2 and phi 712. Subcloning and deletion analysis of the recombinant plasmid pCG1 defined a 3.7-kb EcoRV-XbaI fragment as encoding phage insensitivity. DNA sequence analysis of this region revealed the presence of a single complete ORF. The introduction of a frameshift mutation at the unique BglII site within this ORF disrupted the phage resistance phenotype, confirming that this ORF is responsible for the observed phage insensitivity. The mechanisms encoded by pPG01 and pCG1 in L. lactis subsp. lactis MG1614 conformed to the criteria defining abortive infection and were designated AbiE and AbiF, respectively. Analysis of the phage DNA content of phi 712-infected hosts containing AbiF demonstrated that it inhibited the rate of phage DNA replication, while AbiE had little effect on phage DNA replication, suggesting a later target of inhibition. The predicted protein product of abiF shows significant homology to the products of two other lactococcal abortive infection genes, abiD and abiD1.

Bacteriophage T7 morphogenesis and gene 10 frameshifting in Escherichia coli showing different degrees of ribosomal fidelity

Bacteriophage T7 infection has been studied in Escherichia coli strains showing both increased and decreased ribosome fidelity and in the presence of streptomycin, which stimulates translational misreading, in an effort to determine effects on the apparent programmed translational frameshift that occurs during synthesis of the gene 10 capsid protein. Quantitation of the protein bands from SDS-PAGE failed to detect any significant effects on the amounts of the shifted 10B protein relative to the in-frame 10A protein under all fidelity conditions tested. However, any changes in fidelity conditions led to inhibition of phage morphogenesis in single-step growth experiments, which could not be accounted for by reduced amounts of phage protein synthesis, nor, at least in the case of decreased accuracy, by reduced amounts of phage DNA synthesis. Reduction in phage DNA synthesis did appear to account for a substantial proportion of the reduction in phage yield seen under conditions of increased accuracy. Similar effects of varying ribosomal fidelity on growth were also seen with phage T3, and to a lesser extent with phage T4. The absence of change in the high-frequency T7 gene 10 frameshift differs from earlier reports that ribosomal fidelity affects low-frequency frameshift errors.

Mutants of bacteriophage T7 that escape F restriction

Mutants of bacteriophage T7 that escape F restriction have been isolated. Two mutations in gene 10, which codes for the capsid protein, and one mutation in gene 1.2 are required for these phages to grow on F-containing strains. The products of these two genes are the two targets of the exclusion system; the presence of either wild-type product results in an abortive infection. Phages that grow normally in male hosts still lead to membrane dysfunction and nucleotide efflux from the infected cell. This type of membrane damage and the abortive infection are therefore separable phenomena.

A simple method of distinguishing the bacterial viruses T3 and T7, and a critical reevaluation of their heterologous and homologous exclusion

A method is presented allowing a clear distinction between bacterial viruses T3 and T7 by plating on selectively permissive host cells. The indicator strains are Escherichia coli cells containing either cloned pif genes (exclusively permissive for T3) or the EcoRV DNA restriction system (permissive only for T7): The efficiencies of plating of the two phages on these hosts differ by more than 8 orders of magnitude. This method was applied to reinvestigate the controversial question of mutual exclusion between T3 and T7. Under single-burst conditions, about 50% of coinfected cells (permissive for both viruses) produced T3 and T7 progeny while about 25% reproduced only T3 and about 25% only T7. The burst size of co-infected cells was slightly reduced, compared to controls infected with only one virus type. Homologous exclusion among T3 phages was also not seen; rather, there was a gene dosage effect: T3-encoded RNA polymerase activity as well as T3-specific RNA synthesis increased proportionally to the multiplicity of infection (2.5-20 plaque-forming units/cell).