Adaptation Leads to Loss of a Regulatory Site Affecting mRNA Levels and Fitness

Abstract

Adaptive substitutions in regulatory regions of viruses often lead to problems. Our goal is to investigate the contribution of substitutions in regulatory regions to the adaptive evolution of a phage, ϕX174, to its host, Escherichia coli at different temperatures. During experiments where phages were grown in chemostats, the J terminator was deleted, and this deletion spread throughout the phage population. Phenotypic performance and RNA production at 37°C and 42°C was compared between a mutant strain whose J terminator was deleted and an isogenic wild type strain. The results showed that the deletion strain has a faster doubling rate but similar time to lysis as the wild type strain at 42°C, while they have similar doubling rate and time to lysis at 37°C. Capsid assembly assays showed that the deletion strain lags behind the wild type strain in producing virions at 37°C. At both temperatures, the amount of RNA for gene F relative to the amount of RNA for gene D is greater for the deletion strain than the wild type strain. Northern blot analysis showed that the transcript from gene D to gene J decreased after deletion of the J terminator. Based upon the phenotypic assays, the greater amount of RNA for gene F appears to be beneficial at 42°C. Our results suggest that the J terminator plays an important role in adaptive evolution.