RESUMEN
The cyclic AMP receptor protein (CRP) complexed with cyclic AMP (CRP.cAMP) regulates expression of many genes by binding to sites at or near many promoters of Escherichia coli. The regulatory effect of CRP.cAMP was studied by in vitro transcription assay with lacUV5 promoter derivatives that have the CRP binding site at different locations (-56 to -69 from the transcription start site of lacUV5 promoter) upstream of the promoter. The CRP binding site itself influenced differently on the promoter activity depending on the distances from the promoter. Depending on the helix phasing of the CRP.cAMP relative to RNA polymerase CRP.cAMP activated, repressed or had no effect on the promoter. These results imply that a regulator is not a dedicated protein for repression or activation but that any regulator may have a potential of dual functionalities when it is under appropriate condition.