Ubiquitin thioesterase component of the regulatory network controlling carbon source utilization through ubiquitination and deubiquitination involving creA, creB, creC, creD and acrB. Deubiquitinates the creA catabolic repressor and the quinate permease qutD. Also plays a role in response to carbon starvation and the control of extracellular proteases activity. Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). Interacts with creA, creC and qutD. Belongs to the peptidase C19 family.