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

Abstract

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).

Replication of viruses in a growing plaque: a reaction-diffusion model

“…For a lag time of 13 min (21) one may calculate k2 equal to 6/h. Yields (Y) under optimal conditions are -200 active phages per infected host (22). Assuming the equilibrium adsorption constant for phage Ti holds for T7, k1/k-l is 3 x 10-8 cm3 (23).…”

Antiserum inhibition of propagating viruses

“…Shown in Figure 4 is a one-parameter fit of our theoretical model (equation 9) to the data, where p and [Ā ] i are plotted on the y and x axes, respectively. The free parameter is the ratio of the reaction rate constants multiplied by the reference antibody concentration ([A] io ⅐ k 2 /k 1 ), and it is found to be 2.0 × 10 −6 for a yield of 200 progeny per infected host (Kruger, 1988). Within experimental error, the model captures the overall inhibitory effect of the antiserum on the plaque propagation rate.…”