Expression of the genes in the locus of enterocyte effacement (LEE) in enterohaemorrhagic Escherichia coli is primarily coordinated by expression of the LEE1 operon. GrlA is a LEE-encoded transcription regulator that has been proposed to be involved in the regulation of expression of the LEE1 operon. We describe a simple plasmid-based system to investigate the LEE1 operon regulatory region and to study GrlA-dependent effects. We report that GrlA can activate transcription initiation at the LEE1 P1 promoter by binding to a target located within the 18 base pair spacer between the promoter 10 and 35 elements, which were defined by mutational analysis. Shortening this spacer to 17 base pairs increases P1 promoter activity and short-circuits GrlA dependent activation. Hence, at the P1 promoter, the action of GrlA resembles that of many MerR family transcription activators at their target promoters
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