Role of small subunit (IlvN polypeptide) of acetohydroxyacid synthase I from Escherichia coli K-12 in sensitivity of the enzyme to valine inhibition.

Most of the coding sequence for the IlvN polypeptide subunit of acetohydroxyacid synthase I was deleted from the ilvB+ ilvN+ plasmid pTCN12 by in vitro methods. Several ilvB+ delta ilvN derivatives of pTCN12 were identified among transformants of a strain otherwise lacking any acetohydroxyacid synthase. Deletion derivatives produced an enzymatically active IlvB polypeptide, as shown by the Ilv+ phenotype of transformed cells and by immunologic and enzymatic assays. However, whereas the growth of pTCN12 transformants was sensitive to valine inhibition, growth of the ilvB+ delta ilvN transformants was relatively resistant. Moreover, in vitro analyses confirmed that both acetolactate and acetohydroxybutyrate synthesis in extracts of the ilvB+ delta ilvN transformants was resistant to valine inhibition, in comparison with that in extracts of pTCN12 transformants or with that catalyzed by purified acetohydroxyacid synthase I. The IlvN polypeptide had a minimal effect, if any, on IlvB polypeptide accumulation as measured by immunoprecipitation, but its absence resulted in a greater than 10-fold reduction in enzyme specific activity.