In this report a modification to the in vivo footprinting assay is described. The method includes dimethyl sulfate treatment of whole yeast cells, followed by reiterative primer extension of the methylated genomic DNA using Taq DNA polymerase. Under appropriate reaction conditions chain extension terminates opposite a methylated purlne when Taq DNA polymerase encounters a modified adenlne or guanine. The procedure was used to examine, in vivo DNA-protein contacts over the upstream activation site (UAS) of the Saccharomyces cerevlslae PYK gene. in vivo analysis, using isogenic strains of yeast and Escherichla coli transformed with plasmid DNAs, confirmed the binding of both the frens-actlng factor RAP1 and the transcriptlonal activator GCR1 to cis-acting recognition sites located within the PYK UAS element.