Question

(True Story) In 1952, the world’s first nuclear reactor meltdown took place at Canada’s Chalk River experimental nuclear station, with a significant release of radioactivity to the surrounding environment. (A U.S. Navy nuclear Engineer named Jimmy Carter participated in the cleanup). As part of a study of the biological effects of the meltdown, microbiologists isolated E.coli strains for analysis. Eventually, one of them was found to have a mutation in the gene encoding a tRNA-tyr. The anticodon of this tyr tRNA was changed from 3’-AUG-5’ to 3’-AUC-5’.

This mutation makes it so that protein synthesis is terminated early - the UAG codon (which was formerly AUG) leads to termination rather than continued synthesis of the protein.

Bacteriophage T4 mutants that could not grow on (make plaques on) wild-type E. coli were found to be able to make plaques on the “Chalk River” strain (which has a mutation that causes early termination in the e. coli cells). T4 mutations of this phenotype (grew on the CR strain but not W.T. E coli) were not limited to a particular gene or pathway but could be found for nearly all the complementation groups of T4. What do you think is the nature of these T4 mutations, and why can phage carrying them grow on the CR strain but not the wild-type strain?

Answer 1

The chalk river mutant strain has a mutation that leads to early termination of proteins. Almost all the proteins have a tyrosine amino acid. Hence most of the proteins in the chalk river bacteria will have truncated proteins with reduced or altered function. This can have far reaching effects like reduced replication rate, cell division efficiency, reduced motility and nutrient utilisation...etc.
The phage T4 can form plaques on chalk river strain and not on the wild type E.coli. Additionally, all the mutants of T4 can infect the chalk river strain , so it is not specific to a gene or a protein.
Therefore we can conclude that the phage was able to form plaques on plaques because of faulty and trucntated proteins on the chalk river strain which made them susceptible to phage infections.