Ribosome-deficient plastids in the analysis of plastid RNA metabolism
Hess, W.R., Vogel, J., Hübschmann, T., Börner, T.
Humboldt University Berlin, Institute of Biology, Chausseestr. 117, D-10115 Berlin, Germany
Transcription and maturation of RNA in plastids depends on the co-ordinated activity of chloroplast- and nuclear-encoded components. RNA metabolism was studied in plastids of the barley albostrians mutant as a model system. White leaf areas of this mutant lack all plastid encoded proteins due to a deficiency in the chloroplast ribosomal apparatus. In these plastids, transcriptional activity is restricted to a nuclear-encoded phage-type RNA polymerase. Processing of primary transcripts shows distinct defects. Several subgroup IIA introns remain unspliced, whereas splicing of subgroup IIB introns proceeds normally. Transcript 5' and 3' trimming and cleavage is not affected by the ribosome deficiency. In the case of the trnK gene, which is split by a 2.5 kb intron, that remains unspliced, the mature 5' end is efficiently generated by an RNase P activity. This intron is located in the tRNA anticodon loop and contains matK, a putative maturase gene. Due to the splice defect of trnK-matK transcripts, this gene is supposed to encode one of several essential chloroplast splicing factors. Translation of matK seems to occur from released circular intron RNA or from stable precursor RNA since monocistronic matK mRNA is not detectable. What is more, matK transcripts are modified at a first codon base C-to-U RNA editing. The resulting H->Y substitution restores a sequence motif within the X domain that is present in maturases of yeast and plant mitochondria and in Lactococcus ltrA. It may be directly involved in the proposed RNA binding activity of the domain X. RNA editing of matK transcripts is not affected by the albostrians phenotype. This is in agreement with our observations of RNA editing in several other chloroplast transcripts, as being independent of the presence of ribosomes, translation and splicing.
| LOCATION |
DATE |
TIME |
| Lecture Hall I |
Thursday, April 9 |
05:30 pm |