When the mutant 1 has point mutation at-10 region, the promoter can bind at-35 consensus sequence and can initiate transcription. Similarly in mutant 2, point mutation is occurred at-35 region and promoter can bind at -10 region to initiate
transcription. In mutant 3, two point mutations had occurred at both-10 and-35 regions of consensus
sequences and promoter cannot bind at these mutated regions. Therefore,transcription does not
take place and mutant 3 could not make serine.