In a lac operon, a nonsense mutation in the gene encoding beta-galactosidase was found to interfere with the expression of downstream permease and transacetylase genes. Which one of the following may explain this observation most appropriately?
1
polar effect of the mutation
2
trans-effect of the mutation
3
Binding of the release factor to the nonsense codon prevents translation of the downstream cistrons
4
Formation of a stem-loop structure in the upstream cistron prevents translation of downstream cistrons.