Molecular Forms of the Chloroplast Leucyl-tRNA Synthetase of Euglena gracilis Z Strain

N-(phosphonomethyl)glycine (glyphosate) tolerance in Euglena gracilis acquired by either overproduced or resistant 5-enolpyruvylshikimate-3-phosphate synthase

Photoautotrophic cells of Euglena gracilis can be adapted to N-(phosphonomethyl)glycine (glyphosate) by cultivation in media with progressively higher concentrations of the herbicide. Two different mechanisms of tolerance to the herbicide were observed. One is characterized by the overproduction and 40-fold accumulation of the target enzyme. 5-enolpyruvylshikimate-3-phosphate synthase, in cells adapted to 6 mM N-(phosphonomethyl)glycine. The other is connected with a herbicide-insensitive enzyme. No evidence was obtained for the involvement of the putative multifunctional arom protein previously reported to be involved in the biosynthesis of aromatic amino acids in Euglena. Cells adapted to N-(phosphonomethyl)glycine excreted shikimate and shikimate 3-phosphate into the medium: the amounts depended on the actual concentration of the herbicide. Two-dimensional gel electrophoresis and determination of 5-enolpyruvylshikimate-3-phosphate synthase activity in crude extracts, as well as after separation by non-denaturing gel electrophoresis, revealed that the overproduction of the enzyme in adapted cells correlates with the accumulation of a 59-kDa protein. Overproduction of this 59-kDa protein resulted from a selectively increased level of a mRNA coding for a 64.5-kDa polypeptide which appeared in adapted cells, as shown by cell-free translation in the wheat germ system. In contrast to this quantitative, adaptive type of tolerance, the second mechanism causing tolerance to N-(phosphonomethyl)glycine in the Euglena cell line NR 6/50 was probably related to a qualitatively altered 5-enolpyruvylshikimate-3-phosphate synthase, which could not be inhibited by even 2 mM N-(phosphonomethyl)glycine in vitro. In agreement with this observation, the putatively mutated cell line excreted neither shikimate nor shikimate 3-phosphate into the growth medium containing N-(phosphonomethyl)glycine, even if cultivated in the presence of 20 mM or 50 mM N-(phosphonomethyl)glycine.

In-vitro transport of chloroplast proteins in a homologousEuglena system with particular reference to plastid leucyl-tRNA synthetase

In-vitro translations of total or polyadenylated RNA from chemoorganotrophic and photoautotrophicEuglena gracilis showed no substantial differences in the polypeptide patterns of the two cell types. By contrast, the corresponding patterns of in-vivo labelling indicated that posttranscriptional control of abundant cellular proteins occurred in illuminated cells. This type of control was confirmed forEuglena chloroplast proteins of cytoplasmic origin. The posttranslational transport of in-vitro-formed polypeptides into homologous chloroplasts allowed the plastid-targeted proteins to be recognized. Estimations of the amounts of in-vitro-translated polypeptides showed that the mRNA levels for nuclearencoded chloroplast proteins were almost constant throughout chloroplast development inEuglena. The import of the in-vitro-translation products into the chloroplasts was demonstrated (i) by kinetic determination of radioactivity increasing inside and decreasing outside the organelles, (ii) by autoradiographic analysis of the transported translation products among two-dimensionally separated chloroplast proteins, and (iii) by autoradiographic estimation of the decrease in labelled polypeptides in the incubation medium. After 60 min of incubation in the presence of chloroplasts, about 20% of the in-vitro-labelled translation products, corresponding to 30 protein spots, were found to be redistributed into the organelles. Low-abundance chloroplast leucyl-tRNA synthetase (LeuRS) was detected immunologically among the in-vitro translation products as a precursor protein (pre-LeuRS) of 112 kilodaltons (kDa), which was transported into chloroplasts where it was present in the form of the 105-kDa mature enzyme. Processing of pre-LeuRS also seemed to occur using wheat-germ translation system; however, the mature enzyme was not sequestered intoEuglena chloroplasts. Our results indicate that a nuclear-encoded chloroplastic aminoacyl-tRNA synthetase, the product of a low-abundance mRNA, is transported into and processed in chloroplasts in vitro.