UASt regulates expression of a Ubiquitin-eGFP chimera with a methionine and a linker sequence GKLGRQ between the ubiquitin- and eGFP-coding sequences. Upon proteolytic cleavage (deubiquitination), a stabilizing amino acid (methionine) is exposed at the N-terminus of the GFP moiety, thereby acting as a stable GFP reporter for activity of the N-end rule pathway.