Elongation factors for chloroplast and mitochondrial protein synthesis in Chlorella vulgaris

Arabidopsis thaliana mitochondrial EF-G1 functions in two different translation steps

Translation elongation factor G (EF-G) in bacteria catalyses the translocation of transfer RNA on ribosomes in the elongation step as well as dissociation of post-termination state ribosomes into two subunits in the recycling step. In contrast, the dual functions of EF-G are exclusively divided into two different paralogues in human mitochondria, named EF-G1mt for translocation and EF-G2mt for ribosomal dissociation. Many of the two eukaryotic EF-G paralogues are phylogenetically associated with EF-G1mt and EF-G2mt groups. However, plant paralogues are associated with EF-G1mt and plastid EF-G, not with EF-G2mt. In this study, we phylogenetically and biochemically characterized Arabidopsis thaliana EF-G1mt (AtEF-G1mt) to clarify the factor responsible for the dissociation of ribosomes in plant mitochondria. We showed that eukaryotic EF-G1mts form one monophyletic group separated from bacterial EF-G and are classified into five sister groups. AtEF-G1mt is classified into a different group from its human counterpart. We also demonstrated that AtEF-G1mt catalyses both translocation and ribosomal dissociation, unlike in humans. Meanwhile, AtEF-G1mt is resistant to fusidic acid, an inhibitor of bacterial EF-G. Here, we propose that the functional division is not necessarily conserved among mitochondriate eukaryotes and also that EF-G1mt in organisms lacking EF-G2mt functions in two steps, similar to conventional bacterial EF-G.

Purification and characterization of the mitochondrial translocase from Euglena gracilis

The Euglena gracilis mitochondrial protein biosynthetic elongation factor G (EF-Gmt) has been purified in four steps to greater than 50% homogeneity by use of a fusidic acid affinity procedure and conventional chromatographic techniques. The purification scheme results in 1100-fold purification with about 3% recovery of the total EF-G activity present in the postribosomal supernatant prepared from whole cell extracts. E. gracilis EF-Gmt has an approximate molecular weight of 76,000, comparable to that observed for procaryotic translocases. As is the case for other translocases which have been examined, pretreatment of E. gracilis EF-Gmt with N-ethylmaleimide results in a loss of polymerization activity, indicating a role for an essential cysteine residue in catalytic activity. GDP partially protects EF-Gmt from N-ethylmaleimide inactivation. E. gracilis EF-Gmt functions well on both Escherichia coli and E. gracilis chloroplast ribosomes, but has negligible activity on wheat germ cytoplasmic ribosomes. In this respect, it differs significantly from the mitochondrial translocase of yeast which has very little activity on chloroplast ribosomes. When assayed on E. coli ribosomes, E. gracilis EF-Gmt is sensitive to the steroid antibiotic, fusidic acid, at levels similar to that required for inactivation of E. coli EF-G. It is less sensitive than E. gracilis chloroplast EF-G, and is more sensitive than Bacillus subtilis EF-G. When assayed on E. gracilis chloroplast ribosomes, the same trends in sensitivities are observed, although the exact level of fusidic acid required for inactivation is slightly altered.

Altered methionyl-tRNA synthetase in a Spirulina platensis mutant resistant to ethionine

Compared with the parental strain, a Spirulina platensis mutant that is resistant to ethionine incorporated methionine into protein at a reduced rate, whereas ethionine incorporation was practically nil. The methionyl-tRNA synthetase present in crude extracts from the resistant strain showed a reduced affinity for methionine and ethionine.

Chloroplast elongation factors are synthesized in the chloroplast

The elongation factor G (EF-Gchl) and elongation factor Tu (EF-Tuchl) present in spinach chloroplasts become labelled when isolated chloroplasts are incubated in the light with radioactive methionine. EF-Gchl and EF-Tuchl account for approximately 0.04% and 0.2% respectively of the total radioactivity incorporated by isolated organelles.

Purification of the elongation factors present in spinach chloroplasts

Elongation factor G (EF-Gchl) and elongation factor Tu (EF-TUchl) have been purified from isolated spinach chloroplasts. On polyacrylamide gel electrophoresis the purifed proteins appear to be at least 70% pure. The molecular weight has been estimated to be 77000 and 45500 for EF-Gchl and EF-TUchl respectively. Chloroplast elongation factor T (EF-Tchl) has been only partially purified. Gel electrophoresis under non-denaturing and denaturing conditons indicate that EF-T-chl is most probably composed of two polypeptides, one of which has an electrophoretic mobility identical to that of EF-TUchl. EF-TUchl appears to represent approximately 7% of the chloroplast soluble protein while EF-Gchl accounts for less than 1%. Just as in the case of the bacterial factors, EF-TUchl appears to be in excess as compared to EF-Gchl. Although no data were obtained on the concentration of EF-Tchl, it may be assumed that the three elongation factors represent approximately 10% of the chloroplast soluble protein.