A PROMOTER RELAY MECHANISM FOR SEQUENTIAL GENE ACTIVATION

The effect of DNA supercoiling on gene expression is dependent not onl y on specific genes but also on the sequence context of the genes. Thi s position-dependent supercoiling effect on gene activation is best il lustrated in the study of the suppression of the leu-500 mutation of t he leuABCD operon in a Salmonella typhimurium topA mutant. In this com munication, we report a novel promoter relay mechanism whereby several genes are sequentially expressed in a position-dependent manner: the ilvIH promoter (pilvIH) activates a cryptic leu0 promoter (pleu0) loca ted between the two divergently arrayed ilvIH and leu-500 promoters. B oth the cis-acting pleu0 activity and the trans-acting Leu0 protein ar e necessary for subsequent activation of the leu-500 promoter (pleu-50 0). Furthermore, pleu0 can be functionally replaced with the inducible tac promoter (ptac) for leu-500 activation, suggesting that transcrip tion-driven DNA supercoiling underlies the relay mechanism. This is th e first example of several related genes communicating via a promoter relay mechanism for their coordinated expression.