An upstream activating sequence or upstream activation sequence (UAS) is a cis-acting regulatory sequence. It is distinct from the promoter and increases the expression of a neighbouring gene.
In genetics, an enhancer is a short (50-1500 bp) region of DNA that can be bound by proteins (activators) to increase the likelihood that transcription of a particular gene will occur.
I find no difference between them, are they synonymous ?
Due to its essential role in activating transcription, the upstream activating sequence is often considered to be analogous to the function of the enhancer in multicellular eukaryotes.
There appears to exist another kind of regulatory sequence, downstream activating element.I think it's another synonym for enhancer located downstream the transcription start site.