Methods and materials useful for modulating the sensitivity of cells to an inhibitor of acetohydroxyacid synthase (AHAS) are disclosed. For example, nucleic acid molecules encoding AHAS large subunits are disclosed as well as methods for using such nucleic acid molecules to transform microbial cells and plant cells, and to confer modulated sensitivity to AHAS-inhibiting compounds onto such cells. Further provided are materials and methods useful for modulating growth, development, activity, and characteristics of host cells and organisms.