The nucleotide sequences of the genes coding for tRNAArgUCU, tRNAArgACG and tRNAAsnGUU on Spirodela oligorhiza chloroplast DNA

ERIL1, the plant homologue of ERI-1, is involved in the processing of chloroplastic rRNAs

Proteins belonging to the enhancer of RNA interference-1 subfamily of 3'-5' exoribonucleases participate in divergent RNA pathways. They degrade small interfering RNAs (siRNAs), thus suppressing RNA interference, and are involved in the maturation of ribosomal RNAs and the degradation of histone messenger RNAs (mRNAs). Here, we report evidence for the role of the plant homologue of these proteins, which we termed ENHANCED RNA INTERFERENCE-1-LIKE-1 (ERIL1), in chloroplast function. In vitro assays with AtERIL1 proved that the conserved 3'-5' exonuclease activity is shared among all homologues studied. Confocal microscopy revealed that ERL1, a nucleus-encoded protein, is targeted to the chloroplast. To gain insight into its role in plants, we used Nicotiana benthamiana and Arabidopsis thaliana plants that constitutively overexpress or suppress ERIL1. In the mutant lines of both species we observed malfunctions in photosynthetic ability. Molecular analysis showed that ERIL1 participates in the processing of chloroplastic ribosomal RNAs (rRNAs). Lastly, our results suggest that the missexpression of ERIL1 may have an indirect effect on the microRNA (miRNA) pathway. Altogether our data point to an additional piece of the puzzle in the complex RNA metabolism of chloroplasts.

PRBP plays a role in plastid ribosomal RNA maturation and chloroplast biogenesis in Nicotiana benthamiana

In the present study, we investigated protein characteristics and physiological functions of PRBP (plastid RNA-binding protein) in Nicotiana benthamiana. PRBP fused to green fluorescent protein (GFP) localized to the chloroplasts. Recombinant PRBP proteins bind to single-stranded RNA in vitro, but not to DNA in a double- or a single-stranded form. Virus-induced gene silencing (VIGS) of PRBP resulted in leaf yellowing in N. benthamiana. At the cellular level, PRBP depletion disrupted chloroplast biogenesis: chloroplast number and size were reduced, and the thylakoid membrane was poorly developed. In PRBP-silenced leaves, protein levels of plastid-encoded genes were significantly reduced, whereas their mRNA levels were normal regardless of their promoter types indicating that PRBP deficiency primarily affects translational or post-translational processes. Depletion of PRBP impaired processing of the plastid-encoded 4.5S ribosomal RNA, resulting in accumulation of the larger precursor rRNAs in the chloroplasts. In addition, PRBP-deficient chloroplasts contained significantly reduced levels of mature 4.5S and 5S rRNAs in the polysomal fractions, indicating decreased chloroplast translation. These results suggest that PRBP plays a role in chloroplast rRNA processing and chloroplast development in higher plants.

Cotranscription of 5S rRNA-tRNA(Arg)(ACG) from Brassica napus chloroplasts and processing of their intergenic spacer

S1 mapping showed that at least a significant portion of the 5S rRNA and tRNA(Arg)(ACG) is co-transcribed in canola chloroplast, making trnR the last gene transcribed in an operon of which the final sequence is 5'-16S-tRNA(Ile)-tRNA(Ala)-23S-4.5S-5S-tRNA(Arg)-3'. Various RNA termini representing RNA processing sites at several parts of the 5S rRNA-tRNA(Arg) area were detected. This gene spacer is substantially conserved among various species compared here, and a secondary structure model for this chloroplast region in canola applies to other plant sequences. The conservation of this intergenic sequence suggests a functional role, possibly by providing recognition structures for endogenous RNases involved in its maturing process.

Chloroplast ribosomes and protein synthesis

Consistent with their postulated origin from endosymbiotic cyanobacteria, chloroplasts of plants and algae have ribosomes whose component RNAs and proteins are strikingly similar to those of eubacteria. Comparison of the secondary structures of 16S rRNAs of chloroplasts and bacteria has been particularly useful in identifying highly conserved regions likely to have essential functions. Comparative analysis of ribosomal protein sequences may likewise prove valuable in determining their roles in protein synthesis. This review is concerned primarily with the RNAs and proteins that constitute the chloroplast ribosome, the genes that encode these components, and their expression. It begins with an overview of chloroplast genome structure in land plants and algae and then presents a brief comparison of chloroplast and prokaryotic protein-synthesizing systems and a more detailed analysis of chloroplast rRNAs and ribosomal proteins. A description of the synthesis and assembly of chloroplast ribosomes follows. The review concludes with discussion of whether chloroplast protein synthesis is essential for cell survival.

Chloroplast ribosomes and protein synthesis

Consistent with their postulated origin from endosymbiotic cyanobacteria, chloroplasts of plants and algae have ribosomes whose component RNAs and proteins are strikingly similar to those of eubacteria. Comparison of the secondary structures of 16S rRNAs of chloroplasts and bacteria has been particularly useful in identifying highly conserved regions likely to have essential functions. Comparative analysis of ribosomal protein sequences may likewise prove valuable in determining their roles in protein synthesis. This review is concerned primarily with the RNAs and proteins that constitute the chloroplast ribosome, the genes that encode these components, and their expression. It begins with an overview of chloroplast genome structure in land plants and algae and then presents a brief comparison of chloroplast and prokaryotic protein-synthesizing systems and a more detailed analysis of chloroplast rRNAs and ribosomal proteins. A description of the synthesis and assembly of chloroplast ribosomes follows. The review concludes with discussion of whether chloroplast protein synthesis is essential for cell survival.

Structure and organization of Marchantia polymorpha chloroplast genome. III. Gene organization of the large single copy region from rbcL to trnI(CAU)

The nucleotide sequence (25,320 base-pairs) of a part of the large single-copy region of chloroplast DNA from the liverwort Marchantia polymorpha was determined. This region encodes putative genes for four tRNAs, isoleucine tRNA(CAU), arginine tRNA(CCG), proline tRNA(UGG) and tryptophan tRNA(CCA); eight photosynthetic polypeptides, the large subunit of ribulose bisphosphate carboxylase/oxygenase (rbcL), 51,000 Mr photosystem II chlorophyll alpha apoprotein (psbB), apocytochrome b-559 polypeptides (psbE and psbF), 10,000 Mr phosphoprotein (psbH), cytochrome f preprotein (petA), cytochrome b6 polypeptide (petB), and cytochrome b6/f complex subunit 4 polypeptide (petD); 13 ribosomal proteins (L2, L14, L16, L20, L22, L23, L33, S3, S8, S11, S12, S18 and S19); initiation factor 1 (infA); ribosome-associating polypeptide (secX); and alpha subunit of RNA polymerase (rpoA). Functionally related genes were located in several clusters in this region of the genome. There were two ribosomal protein gene clusters: rpl23-rpl2-rps19-rpl22-rps3-rpl16-+ ++rpl14-rps8-infA-secX-rps11-rpoA, with a gene arrangement similar to that of the Escherichia coli S10-spc-alpha operons, and the rps12'-rpl20-rps18-rpl33 cluster. There were gene clusters encoding photosynthesis components such as the psbB-psbH-petB-petD and the psbE-psbF clusters. Thirteen open reading frames, ranging in length from 31 to 434 amino acid residues, remain to be identified.

The region distal to the rRNA operon from chloroplasts of maize contains genes coding for tRNA(Arg)(ACG) and tRNA (Asn)(GUU)

The nucleotide sequence distal to the rRNA operon from maize chloroplasts has been analyzed. It contains genes coding for tRNA(Arg)(ACG) and tRNA(Asn)(GUU). The tRNA(Arg)(ACG) gene, which is separated from the last rRNA gene of the rRNA operon, the 5S rDNA, by an intergenic region of 252 bp, has the same orientation as the rRNA operon. By S1 and primer extension mapping, the existence of transcripts from the entire 5S rDNA/tDNA(Arg)(ACG) intergenic region can be demonstrated. It is, therefore, concluded that tRNA(Arg)(ACG) represents a trailer tRNA which is cotranscribed with 5S rRNA as part of the primary rRNA transcript. The tDNA(Asn)(GUU), which is separated from tDNA(Arg)(ACG) by an intergenic region of 253 bp, has the opposite orientation with respect to the rRNA operon; it, therefore, represents a separate transcriptional unit whose promoter remains to be located. It is proposed that the two tRNA genes possess a common terminator region, which functions in both directions of transcription.

Nucleotide sequences of transfer RNA genes in the Pisum sativum chloroplast DNA

Eight transfer RNA (tRNA) genes which were previously mapped to five regions of the Pisum sativum (pea) chloroplast DNA (ctDNA) have been sequenced. They have been identified as tRNA(Val)(GAC), tRNA(Asn)(GUU), tRNA(Arg)(ACG), tRNA(Leu)(CAA), tRNA(Tyr)(GUA), tRNA(Glu)(UUC), tRNA(His)(GUG), and tRNA(Arg)(UCU) by their anticodons and by their similarity to other previously identified tRNA genes from the chloroplast DNAs of higher plants or from E. gracilis. In addition,two other tRNA genes, tRNA(Gly) (UCC) and tRNA(Ile)(GAU), have been partially sequenced. The tRNA genes are compared to other known chloroplast tRNA genes from higher plants and are found to be 90-100% homologous. In addition there are similarities in the overall arrangement of the individual genes between different plants. The 5' flanking regions and the internal sequences of tRNA genes have been studied for conserved regions and consensus sequences. Two unusual features have been found: there is an apparent intron in the D-loop of the tRNA(Gly)(UCC), and the tRNA(Glu)(UUC) contains GATTC in its T-loop.

Location and nucleotide sequence of the genes for tobacco chloroplast tRNAArg (ACG) and tRNALeu(UAG)

The location and nucleotide sequence of the genes and flanking regions for tRNAArg(ACG) and tRNALeu(UAG) on tobacco chloroplast DNA have been determined. The gene arrangement is 5S rRNA-260 bp-tRNAArg-581 bp-tRNAAsn-5.2 kbp-tRNALeu. The tRNAArg and tRNALeu genes are expressed in the chloroplasts. The opposite strand of the tRNAArg gene contains a tRNAArg-like sequence. The tRNAArg, tRNAAsn and tRNALeu coding regions are contained in open reading frames.

A transfer RNAArg gene of Pelargonium chloroplasts, but not a 5S RNA gene, is efficiently transcribed after injection into Xenopus oocyte nuclei

We present the primary structure of a chloroplast tRNAArgACG gene of the plant, Pelargonium zonale, and its faithful expression in Xenopus oocyte nuclei. This tRNAArg gene is located 250 bp downstream of a 5S RNA gene within a cloned 5kb long ribosomal DNA segment (Fig. 1). The Pelargonium tRNAArg gene shares 97% and 86% sequence homology with tRNAArgACG genes of Spirodela oligorhiza and Euglena gracilis chloroplasts, respectively, and also extensive homology (70%) with the corresponding gene of E. coli. It lacks an intervening sequence and, like eukaryotic tRNA genes, does not code for the 3' terminal CCA nucleotides. Moreover, the chloroplast tRNAArg gene carries all the sequence elements essential for transcription by vertebrate RNA polymerase III since it is efficiently expressed in Xenopus oocyte nuclei, even in the presence of 1 microgram/ml alpha-amanitin. In Xenopus oocyte nuclei, no transcripts of the chloroplast 5S RNA gene were detected.