Nonsense mutations affecting the his4 enzyme complex of yeast.

The his4 region of yeast contains the information necessary for the catalysis of three steps in the histidine biosynthetic pathway. The three activities specified by this region remain physically associated during gel filtration and ultracentrifugation and after extensive purification. Nonsense mutations in the "operator distal" his4C region lower the molecular weight of the two proximal activities. The proximal activities can, therefore, function without the his4C portion of the protein(s). These observations are compatible with the following three possibilities. The his4 region codes for: (1) an aggregate of three proteins forming a multiprotein complex; (2) a single protein with three catalytic activities; and (3) a single protein which is cleaved to form three proteins which remain associated.