TRANSCRIPTIONAL ACTIVATION OF THE ESCHERICHIA-COLI BGL OPERON - NEGATIVE REGULATION BY DNA STRUCTURAL ELEMENTS NEAR THE PROMOTER

The bgl operon of Escherichia coil is transcriptionally inactive in wi ld-type cells. DNA insertion sequences (IS) constitute a major class o f spontaneous mutations that activate the cryptic bgl promoter. In an attempt to study the molecular mechanism of activation mediated by ins ertion sequences, transcription of the bgl promoter was carried out in vitro. Stimulation of transcription is observed when a plasmid contai ning an insertionally activated bgl promoter is used as a template in the absence of proteins other than RNA polymerase. Deletions that remo ve sequences upstream of the bgl promoter, and insertion of a 1.2 kb D NA fragment encoding resistance to kanamycin, activate the promoter. P oint mutations within a region of dyad symmetry upstream of the promot er, which has the potential to extrude into a cruciform structure unde r torsional stress, also lead to activation, Introduction of a sequenc e with dyad symmetry, upstream of an activated bgl promoter carrying a deletion of upstream sequences, results in a fourfold reduction in tr anscription, These results suggest that the cryptic nature of the bgl promoter is because of the presence of DNA structural elements near th e promoter that negatively affect transcription.