Ubiquitin bacterial (UBact)

UBact is the second ubiquitin analog found in bacteria, the first one being prokaryotic ubiquitin-like protein Pup.

Protein analogous to the eukaryotic ubiquitin was discovered in 2008 and thought to be characteristic of only gram-positive bacteria from the phylum Actinobacteria. UBact is a newly discovered ubiquitin analog that exists in several phyla of gram-negative bacteria.

UBact identification is the resault of my work at the lab of Professor Aaron Ciechanover. I was able to identify UBact because its neighbors in the genome code for proteins homologous to those known to be involved in conjugation and/or degradation of Pup and Pup-tagged substrates in the gram-positive Actinobacteria. Although the ubiquitin analogs UBact and Pup could not be linked by their aminio acid sequence, the homology between their neighboring conjugation and degradation genes indicate that they are distant homologs.

It is uncertain, why we find Ubact/Pup-proteasome system in some bacterial phyla and not in others. Assuming that UBact/Pup have a common bacterial ancestor that existed before the split into gram-positive and gram-negative, we would have expected to see the systems in more "offspring" of that ancestor. In contrast to this expectation, many bacterial phyla including firmicutes (gram-positive) and proteobacteria (gram-negative) do not seem to have this system. Did they lose it, or did they never have it?

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