Evolution of elongation factor G and the origins of mitochondrial and chloroplast forms

• Protein synthesis elongation factor G (EF-G) is an essential protein with central roles in both the elongation and ribosome recycling phases of protein synthesis. Although EF-G evolution is predicted to be conservative, recent reports suggest otherwise. We have characterized EF-G in terms of its molecular phylogeny, genomic context and patterns of amino acid substitution. We find that most bacteria carry a single "canonical" EF-G, which is phylogenetically conservative and encoded in an str operon. However, we also find a number of EF-G paralogs. These include a pair of EF-Gs that are mostly found together and in an eclectic subset of bacteria, specifically delta-proteobacteria, spirochaetes and planctomycetes (the "spd" bacteria). These spdEFGs have also given rise to the mitochondrial factors mtEFG1 and mtEFG2, which probably arrived in eukaryotes before the eukaryotic last common ancestor. Meanwhile, chloroplasts apparently use an α-proteobacterial derived EF-G, rather than the expected cyanobacterial form. The long-term co-maintenance of the spd/mtEFGs may be related to their subfunctionalization for translocation and ribosome recycling. Consistent with this, patterns of sequence conservation and site-specific evolutionary rate shifts suggest that the faster evolving spd/mtEFG2 has lost translocation function, but, surprisingly, the protein also shows little conservation of sites related to recycling activity. On the other hand, spd/mtEFG1, although more slowly evolving, shows signs of substantial remodeling. This is particularly extensive in the GTPase domain, including a highly conserved three amino acid insertion in switch I. We suggest that sub-functionalization of the spd/mtEFGs is not a simple case of specialization for subsets of original activities. Rather the duplication allows the release of one paralog from the selective constraints imposed by dual functionality thus allowing it to become more highly specialized. Thus the potential for fine-tuning afforded by subfunctionalization may explain the maintenance of EF-G paralogs.

Subject headings

NATURVETENSKAP  -- Biologi -- Biologisk systematik (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Biological Systematics (hsv//eng)

Keyword

EF-G

Elongation factor G

organelle

xenology

paralogy

ribosome

translation

Systematics and phylogenetics

Systematik och fylogeni

Biologi med inriktning mot molekylär evolution

Biology with specialization in Molecular Evolution

Publication and Content Type

ref (subject category)

art (subject category)