GroE is a chaperonin folding system consisting of GroEL (Cpn60, a 60 kDa chaperonin), and the smaller co-chaperonin GroES (Cpn10). Many ‘client’ proteins require GroE to fold properly, including several that are essential for cell viability. GroE is found in nearly all bacteria and eukaryotes.
Mollicutes
are the only micro-organisms that lack GroE in almost all cases. Only two clades of
Mollicutes
have retained the ancestral GroE system, or perhaps reacquired one; these exceptions include the family
Acholeplasmataceae
, consisting of the genera
Acholeplasma
and ‘
Candidatus
Phytoplasma’. The role of GroEL in these unique
Mollicutes
is a source of speculation, given how many non-canonical ‘moonlighting’ roles have been ascribed to this protein. GroEL has been suggested to play a role in pathogenesis in plant and animal pathogenic
Mollicutes
by binding to host cells and facilitating invasion. However, in one further layer of exception, the phytopathogenic taxon ‘
Candidatus
Phytoplasma pruni’ (ribosomal group 16SrIII) was reported in 2012 to lack a GroE system. This study confirms the lack of a functional GroE system in 16SrIII by providing two new, high-quality, non-fragmented genome assemblies, as well as a thorough survey of other 16SrIII genomes for genes encoding GroEL/GroES, including those that may not resemble phytoplasma GroEL (i.e. acquired by horizontal gene transfer, HGT). We discuss the implications of a clearly phytopathogenic, invasive group of
Mollicutes
that nevertheless lacks GroE, in light of the presumed role of GroEL for this species. We determined that multiple genomes of 16SrIII contain short, non-functional
groEL
pseudogenes, while most of the reported genomes lack any semblance of a GroE system. Examination of the new assemblies allowed us to rule out HGT as a means of GroE acquisition.