Conformational change in the DNA associated with an unusual promoter mutation in a tRNA operon of Salmonella.

We describe two novel types of mutations which decrease transcription of a tRNA operon in Salmonella typhimurium. One mutation consists of a 3 bp deletion at position -70 from the transcription start site. The second mutation is a single bp insertion between the -10 region and the transcription start site. The region around position -70 shows unusual physical properties. DNA fragments carrying the wild-type version of this region exhibit abnormally low electrophoretic mobility in polyacrylamide gels. This anomalous electrophoretic behavior is corrected if gels are run at high temperature or if the fragments contain the 3 bp deletion at -70. Our results indicate the existence of an unusual DNA conformation, probably involving DNA bending, in the region preceding the tRNA operon promoter. This structure is apparently disrupted by the 3 bp deletion. The transcriptional defect of the -70 mutant suggests a role for the unusual conformation in promoter function.