Isolation and analysis of two Escherichia coli K-12 ilv attenuator deletion mutants with high-level constitutive expression of an ilv-lac fusion operon.

A lysogenizing lambda phage, lambda dilv-lac11, was constructed to carry an ilvD-lac operon fusion. Expression from the phage of the ilvE and lacZ genes is controlled by an intact ilv control region also carried by this phage. Two spontaneous mutants of lambda dilv-lac11 that have high-level constitutive expression of the ilv-lac fusion operon were isolated by growth on a beta-chloroalanine selective medium. The mutants were shown by nucleotide sequence determination to contain large deletions (delta 2216, approximately 1.6 kilobases; delta 2219, approximately 1.9 kilobases), which in both cases remove the proposed ilv attenuator terminator. The rest of the ilv leader and promoter region DNA remains intact in these mutants. Deletion 2216 also removed part of the downstream ilvG gene, whereas delta 2219 extended through the entire ilvG gene into the ilvGE intercistronic region. A possible mechanism of deletion formation is discussed.