Bacterial systems for analyzing ubiquitylated polypeptides

A composition comprising a first and a second expression vector:

• wherein the first expression vector comprises a first origin of replication, a first promoter, a polynucleotide capable of expressing at least one enzyme that is part of a ubiquitin pathway, a polynucleotide encoding a first antibiotic resistance protein, and a polynucleotide encoding a first fragment of a second antibiotic resistance protein;
• wherein the second expression vector comprises, a second origin of replication, a second promoter, a polynucleotide capable of expressing a ubiquitinating enzyme target substrate, a polynucleotide encoding a third antibiotic resistance gene, and a polynucleotide encoding a second fragment of the second antibiotic resistance protein;
• wherein the first, second, and third antibiotic resistance proteins are orthogonal to each other;
• and
• wherein the first and the second origin of replications are not the same.