| Abstract: | The genes governing three of the enzymes of the isoleucine-valine biosynthetic pathway form the operon: operator-ilvA-ilvD-ilvE. The enzymes are: ilvA, l-threonine deaminase; ilvD, dihydroxy acid dehydrase; and ilvE, transaminase B. A nonsense mutation in the ilvD gene (D-ochre) and a nonsense mutation in the ilvE gene (E-amber) affect the properties of the proximal gene product, l-threonine deaminase (TD), in addition to inactivating the enzymes produced by the genes in which the mutations have occurred. The D-ochre mutation causes TD to move in diffusion and gel filtration experiments as though it were 30% smaller than the wild-type enzyme. The E-amber mutation causes TD to move in similar experiments as though it were much larger than the wild-type enzyme. Both mutations completely abolish the sensitivity of TD to l-isoleucine, the normal feedback inhibitor of the wild-type enzyme. The effects of the nonsense mutations on TD can be reversed in three ways: by genetic reversion of the D-ochre mutation; by treatment of the altered enzymes with 3.0 m urea; and by forming a heterozygous diploid, containing the wild-type allele as well as the mutant allele of ilvD or ilvE. The results suggest that the subunits of TD undergo abnormal aggregation in the presence of the partial polypeptides produced by the mutant alleles of ilvD or ilvE; multi-enzyme aggregates in extracts of wild type, however, could not be detected. |
