Let's suppose a DNA mutation changes the consensus at the -35 site in a way that inhibits alpha factor binding. Explain how a mutation could inhibit the binding of alpha factor to the DNA. Look at Figure 12.5 and describe two specific base substitutions you think would inhibit the binding of alpha factor. Explain why your base substitution would have this effect.

-35 sequence -10 sequence +1 Transcribed lac operon TTTACA N17 TATGTT N6 A lacl GCGCAA N17 CATGAT NZ A trp operon TTGACA N17 TTAACT NZ A rrnX TTGTCT N16 TAATAT N7 A recA TTGATA N16 TATAAT NZ A lex A TTCCAA N17 TATACT N6 A tRNAtyr TTTACA N16 TATGAT Consensus TTGACA TATAAT sequence FIGURE 12.5 Examples of -35 and -10 sequences within several E. coli promoters. This figure shows the -35 and -10 sequences for one DNA strand found in seven different E. coli promot- ers. The consensus sequence is shown at the bottom. The spacer regions contain the designated number of nucleotides between the -35 and -10 sequences or between the -10 sequence and the transcriptional start site. For example, N17 means there are 17 nucleotides between the end of the -35 sequence and the beginning of the -10 sequence