Evolution of organelle genomes and protein-synthesizing systems

Chloroplast DNA from the fern Osmunda cinnamomea: physical organization, gene localization and comparison to angiosperm

Chloroplast DNA from the fern Osmunda einnamomea was isolated by a sucrose gradient procedure utilizing PEG to stabilize chloroplasts. Analysis with the restriction endonucleases PvuII, Sacl and BstEII indicates a chloroplast genome size of 144 kb. A physical map of the fragments produced by these three enzymes was constructed by filter hybridizations using purified PvuII fragments as hybridization probes. The Osmunda chloroplast genome is circular and contains an inverted repeat 8-13 kb in size.Gene probes from tobacco, corn and spinach were used to map the positions of six genes on the Osmunda chloroplast chromosome. The 16S and 23S ribosomal RNAs are encoded by duplicate genes which lie within the inverted repeat. Genes for the large subunit of ribulose-1,5-bisphosphate carboxylase, a photosystem II polypeptide, and the alpha and beta subunits of chloroplast coupling factor are located in three different segments of the large single copy region.The Osmunda chloroplast genome is remarkably similar in size, conformation, physical organization, and map positions of known genes, to chloroplast DNA from a number of angiosperms. The major difference between chloroplast DNA from this fern and angiosperms is that the inverted repeat is smaller in Osmunda (8-13 kb) than in angiosperms (22-25 kb).

Evidence that the fragmented ribosomal RNAs of Chlamydomonas mitochondria are associated with ribosomes

The discontinuous and scrambled organization of the small subunit and large subunit rRNA coding regions in Chlamydomonas mitochondrial DNA has been well documented. Our goals were to demonstrate that the small transcripts produced by these coding regions in Chlamydomonas eugametos are assembled into mitochondrial ribosomes and to characterize the sedimentation properties of these ribosomes and their subunits in sucrose gradients. Putative mitochondrial ribosomes (60-66S) and their large (44-50S) and small (35-39S) subunits were identified by slot blot hybridization which sedimented independently of the chloroplast and cytosolic ribosomes. A crude mitochondrial pellet prepared from C. eugametos was enriched for mitochondrial small subunit and large subunit rRNA subfragments thereby providing independent confirmation of the mitochondrial association of these rRNA molecules.