A point mutation in Euglene gracilis chloroplast tRNA{sup Glu} uncouples protein and chlorophyll biosynthesis
- Yale Univ., New Haven, CT (United States)
- Univ. of New York, Stony Brook, NY (United States)
The universal precursor of tetrapyrrole pigments (e.g., chlorophylls and hemes) is 5-aminolevulinic acid (ALA), which in Euglena gracilis chloroplasts is derived via the two-step C{sub 5} pathway from glutamate charged to tRNA{sup Glu}. The first enzyme in this pathway, Glu-tRNA reductase (GluTR) catalyzes the reduction of glutamyl-tRNA{sup Glu} (Glu-tRNA) to glutamate 1-semialdehyde (GSA) with the release of the uncharged tRNA{sup Glu}. The second enzyme, GSA-2, 1-aminomutase, converts GSA to ALA. tRNA{sup Glu} is a specific cofactor for the NADPH-dependent reduction by GluTR, an enzyme that recognizes the tRNA in a sequence-specific manner. This RNA is the normal tRNA{sup Glu}, a dual-function molecule participating both in protein and in ALA and, hence, chlorophyll biosynthesis. A chlorophyll-deficient mutant of E. gracilis (Y{sub 9}ZNaIL) does not synthesize ALA from glutamate, although it contains GluTR and GSA-2,1-aminomutase activity. The tRNA{sup Glu} isolated from the mutant can still be acrylated with glutamate in vitro and in vitro. Furthermore, it supports chloroplast protein synthesis; however, it is a poor substrate for GluTR. Sequence analysis of the tRNA and of its gene revealed a C56 {yields} U mutation in the resulting gene product. C56 is therefore an important identity element for GluTR. Thus, a point mutation in the T loop of tRNA uncouples protein from chlorophyll biosynthesis.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 86537
- Journal Information:
- Proceedings of the National Academy of Sciences of the United States of America, Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Issue: 17 Vol. 91; ISSN PNASA6; ISSN 0027-8424
- Country of Publication:
- United States
- Language:
- English
Similar Records
[The first steps of chlorophyll synthesis: RNA involvement and regulation]
Formation of the chlorophyll precursor. gamma. -aminolevulinic acid in cyanobacteria requires aminoacylation of a tRNA sup Glu species. [Synechocystis]