Cloning of the cDNA for glutamyl-tRNA synthetase from Arabidopsis thaliana

Tetrapyrrole Metabolism in Arabidopsis thaliana

Higher plants produce four classes of tetrapyrroles, namely, chlorophyll (Chl), heme, siroheme, and phytochromobilin. In plants, tetrapyrroles play essential roles in a wide range of biological activities including photosynthesis, respiration and the assimilation of nitrogen/sulfur. All four classes of tetrapyrroles are derived from a common biosynthetic pathway that resides in the plastid. In this article, we present an overview of tetrapyrrole metabolism in Arabidopsis and other higher plants, and we describe all identified enzymatic steps involved in this metabolism. We also summarize recent findings on Chl biosynthesis and Chl breakdown. Recent advances in this field, in particular those on the genetic and biochemical analyses of novel enzymes, prompted us to redraw the tetrapyrrole metabolic pathways. In addition, we also summarize our current understanding on the regulatory mechanisms governing tetrapyrrole metabolism. The interactions of tetrapyrrole biosynthesis and other cellular processes including the plastid-to-nucleus signal transduction are discussed.

Expression and properties of arginyl-tRNA synthetase from jack bean (Canavalia ensiformis)

The coding region for arginyl-tRNA synthetase from jack bean (Canavalia ensiformis) has been sequenced and cloned into the bacterial expression vector pET32a. Transformation of BL21 cells and induction with IPTG results in the high level expression of the protein fused N-terminally with thioredoxin and bearing a His-tag. A substantial proportion of the enzyme is recovered in the soluble fraction of the cell lysate (10 mg per litre cell culture) and can be isolated with metal-affinity technology. The thioredoxin component and the His-tag portion of the fused protein could be removed with thrombin, resulting in a homogeneous product retaining an N-terminal extension of 3.2 kDa compared to the native arginyl-tRNA synthetase. Both full-length fusion and thrombin-treated products proved to be active in aminoacylation, with similar kinetic parameters.

The intracellular location of two aminoacyl-tRNA synthetases depends on complex formation with Arc1p

In yeast, two aminoacyl-tRNA synthetases, MetRS and GluRS, are associated with Arc1p. We have studied the mechanism of this complex formation and found that the non-catalytic N-terminally appended domains of MetRS and GluRS are necessary and sufficient for binding to Arc1p. Similarly, it is the N-terminal domain of Arc1p that contains distinct but overlapping binding sites for MetRS and GluRS. Localization of Arc1p, MetRS and GluRS in living cells using green fluorescent protein showed that these three proteins are cytoplasmic and largely excluded from the nucleus. However, when their assembly into a complex is inhibited, significant amounts of MetRS, GluRS and Arc1p can enter the nucleus. We suggest that the organization of aminoacyl-tRNA synthetases into a multimeric complex not only affects catalysis, but is also a means of segregating the tRNA- aminoacylation machinery mainly to the cytoplasmic compartment.