Five mutations that result in reduced expression of the araBAD operon were cloned onto the plasmid pBR322. The position of each mutation was determined by DNA sequence analysis. Three of the mutations were located in the RNA polymerase binding site of the araBAD promoter. The first, ara-1016, was a onebase-pair deletion at position -35; the second, ara-1036, was a transversion at position -13; the third, ara-1027, was a nine-base-pair deletion from +5 to +13. S1 nuclease mapping showed that

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... tions ara-1016 and ara-1036 greatly reduced transcription and that mutation ara-1027 had little, if any, effect on transcription. Two other mutations resulted from the transposition of the insertion element, IS], downstream from the transcriptional start site of the operon. Molecular mechanisms for all of the mutations are discussed. A J. BACTERIOL. on May 8, 2020 by guest