The role of the 'gearbox' in the transcription of essential genes.

Regulation of transcription occurs at different levels, one being in the presence of sequences specifically recognized by different forms of RNA polymerase, i.e. the promoters. Three different kinds of promoter are defined according, among other things, to their dependence on the growth rate of the cell: the 'house-keeper' promoter of many metabolic genes, the stringent promoter found at several rRNA and ribosomal protein genes, and the 'gearbox' at genes whose products are required at higher relative amounts at lower growth rates. The identified gearbox promoters of Escherichia coli share specific homologies in the -10, -35 and upstream regions. Although there may be different types of gearbox promoters, the -10 sequence of one of these promoters has been found to be essential for functioning as a gearbox. This suggests the existence of specific sigma factors for its transcription. RpoS (KatF) is a likely candidate for being one of these sigma factors. Computer simulation allows us to predict that such sigma factors should, in turn, be expressed following a gearbox mode, which would then imply the existence of self-regulated loops contributing to the expression of some genes of bacterial division.