Microorganisms experience a wide variety of fluctuating condition caused by changes in their surroundings or by shifting metabolic demands. Cells must be able to quantify these changes and concordantly alter expression of gene subsets in a measured manner. A great variety of mechanisms for controlling expression in a response to extracellular and intracellular conditions have been identified (66).Most well-characterized among these are the numerous proteins that regulatory control over transcription initiation (9). The large-scale sequencing of bacterial genomes has led to the identification of hundreds of putative transcription factors per genome. For example, more than 200 transcription factors are predicted to be encoded by the Bacillus subtilis genome (79).Therefore, regulation of transcription initiation is a major mech-anism by Which orchestrate gene expression