[citation needed] Similar to ubiquitin, Pup is attached to specific lysine residues of substrate proteins by isopeptide bonds; this is called pupylation.

[5] The Pup homologs were termed UBact (for Ubiquitin Bacterial), although the distinction has not been proven to be phylogenetically supported by a separate evolutionary origin and is without experimental evidence.

[6] While ubiquitin assumes a highly stable three-dimensional structure in solution,[7] Pup has been shown to belong to the group of intrinsically disordered proteins.

[5] The conjugation-proteasome components neighboring the UBact and Pup loci in these Nitrospirae bacteria show weak similarity and are probably not entirely redundant.

Further analyses of the UBact (and not Pup) locus in Leptospirillum ferrodiazotrophum revealed its existence and extreme conservation across several gram-negative bacterial phyla, as illustrated in figure 3.

While Pup is predominantly found in the gram-positive phylum Actinomycetota, UBact was identified only in gram-negative bacteria from the following five phyla: Nitrospirota, Verrucomicrobiota, Armatimonadota, Nitrospinota, and Planctomycetota.

UBact was also identified in the genomes of several candidatus bacteria, and specifically from the candidate divisions "Acetothermia", "Handelsmanbacteria", "Fraserbacteria", "Terrybacteria", "Poribacteria", "Parcubacteria", and "Yanofskybacteria".

[5] With regard to the amino acid sequence, in difference from Pup and Ubiquitin, UBact does not contain a di-glycine motif at its C-terminus.