You identify a proflavin-generated allele of a gene that produces a 110-amino acid polypeptide rather than the usual 157 - amino acid protein. After subjecting this mutant allele to extensive proflavin mutagenesis, you are able to find a number of intragenic suppressors located in the part of the gene between the sequences encoding the N-terminus of the protein and the original mutation but no suppressors located in the region between the original mutation and the sequences encoding the usual C-terminus of the protein. Why do you think this is the case?

Proflavin generated allele exhibit premature stop codon in it, as a consequence of which the mutant allele produces a protein, that is, short of 47 amino acids, that is, devoid of the original C-terminal region. If a similar mutant is subjected extensively to proflavine compound, it produces mutations all over the DNA.

Because of the existence of a premature stop codon, the intragenic suppressor mutation is witnessed only in between N terminal and the original mutation, not in between the original mutation and the C-terminal.

If the original mutation was a missense or sense mutation, one should have witnessed intragenic suppressor mutation at both the C and N terminal. However, in the given case, because of the existence of the premature stop codon, the intragenic suppressor mutation is witnessed only in between the original mutation and the N-terminal.

Subsequently, post the second round of proflavine exposure, if the original mutation containing chain termination codon is modified to any other codon that encrypts for an amino acid, then one would have witnessed intragenic suppressor mutation bot at the C and the N terminal region of the protein.