Suppression of leu-500 mutation in topA+ Salmonella typhimurium strains. The promoter relay at work.

Suppression of leu-500 mutation in Salmonella typhimurium topA- strains has been one of the most fascinating examples for the DNA supercoiling effect on transcription initiation control. Previous studies have indicated possible involvement of transcription-driven DNA supercoiling in the activation of the leu-500 promoter in topA- strains. Our recent studies have shown that ilvIH transcription activity located 1.9 kilobase pairs upstream is the initial supercoiling signal for leu-500 activation via a promoter relay mechanism. In the present communication, we show that the ilvIH transcription activity-initiated promoter relay can result in leu-500 activation in topA+ strains. In addition, suppression of the chromosomal leu-500 mutation correlates with the transcription activities of ilvIH and leuO rather than the TopA level in the topA+ strain. It appears that the leu-500 suppression in a topA- strain is due to the constant ilvIH transcription activity.