The IT Allele
The IT allele of the ALS gene has a change in the coding region sequence. This was discovered by making a gene library from the DNA of homozygous IT plants and selecting a clone of the IT gene. The gene clone was sequenced and compared to the normal susceptible ALS gene from corn. A single nucleotide substitution in the IT allele was responsible for endowing a corn plant with resistance to ALS herbicides. The nucleotide substitution was in the gene coding region so it changed a codon sequence in the mRNA, coding for a different amino acid in the protein. The amino acid substitution had a minor or negligible effect on the enzyme's ability to perform its normal functions. However, the amino acid substitution changed the site at which the ALS herbicide binds to the enzyme. The ALS herbicide will not interact with the ALS enzyme encoded by the IT allele, giving the plant resistance to the herbicide. This is an example of how a small change in a gene can have a big impact on a plant's phenotype in the right environment.