The upstream activator (UAS) of the yeast PGK gene comprises three different sequence elements. These are 1) a region of strong protein binding called the YFP, 2) three repeats of the motif CTTCC and 3) an essential activator core (AC) sequence that binds the protein RAP1. To assess the function of each of these elements in transcriptional activation we have inserted them individually and in various combinations into a PGK mini-promoter. This comprises only the transcription initiation elements from the PGK promoter and is inactive in the absence of activator sequences. None of the individual sequence elements was capable of activating the mini-promoter. However either the YFP or the CTTCC boxes in conjunction with the AC box resulted in efficient expression. Transcription levels were not however as high as when all three elements were inserted. These data suggest that the efficiency of PGK transcription depends upon the interactions between three different sequences. Furthermore while RAP1 per se is not a transcriptional activator it can associate promiscuously with other factors to create a functional transcription complex.