A ubiquitin promoter drives expression of a proteasome activity reporter protein that consists of an uncleavable ubiquitin moiety (Tag:degron(Ub-G76V)) fused to the N-terminal end of Avic\GFP. This results in the tagged protein becoming a target for poly-ubiquitylation and subsequent degradation by the ubiquitin fusion degradation pathway (details from PMID:10802622). The fusion protein can thus only accumulate when proteasome function is inhibited, and GFP fluorescence can be used as a readout of proteasome activity.