Gene Expression In Plant Mitochondria Research Articles

Analysis of the four cox2 genes found in turnip (Brassica campestris, Brassicaceae) mitochondria

In turnip mitochondria the cox2 gene is partially contained within a 2-kb (kilobase) direct repeat that is the site of homologous recombination events that generate subgenomic molecules. Because of its partial presence within the direct repeat, the turnip cox2 gene exists in four genomic contexts, two on the master chromosome and one on each of the subgenomic circles. In order to gain information on the expression of plant mitochondrial genes that exist in multiple genomic contexts and on plant mitochondrial recombination repeats in general we have characterized all four copies of the turnip 2-kb repeat and the cox2 gene. DNA sequence analysis indicates that the turnip high-frequency recombination repeat is 2424 bp in length. All but the last 58 bp of the cox2 gene is contained within the repeat, resulting in the presence of two intact cox2 genes and two pseudogenes. An intact cox2 gene and a pseudogene are found on the master chromosome (218 kb). An intact cox2 gene is also found on the 135-kb subgenomic circle, while a pseudogene is found on the 83-kb subgenomic circle. The turnip cox2 gene is unique because it lacks the first or 5' intron and contains the second or 3' intron previously found only in the carrot. The turnip intron is a group II intron that has high similarity (94.7%) with the second intron in carrot; also it is in precisely the same position within the gene. Transcript mapping studies using locus-specific probes reveal stable transcripts from the intact genes but not from the pseudogenes.

Translational regulation of gene expression in plants

Translational regulation plays a major role in plant gene expression. Recent advances have been made in understanding how this mechanism of control is directed in the choloroplast as well as in the cytoplasm. Our knowledge of translational regulation in plant mitochondria, however, is limited. Translational control of gene expression in yeast mitochondria suggests that similar mechanisms will also play a significant role in the expression of plant mitochondrial genes.

Cell-Specific Expression of Mitochondrial Transcripts in Maize Seedlings.

Although mitochondria are thought to assume crucial and possibly novel physiological functions during male gametogenesis, it is not known to what extent mitochondrial function is necessary for other aspects of plant development or to what degree the expression of plant mitochondrial genes is subject to cell-specific regulation, particularly during vegetative growth. We have used in situ hybridization to show that extensive differences exist in the levels of mitochondrial RNAs (mtRNAs) among different tissues and among different individual cell types within the same organ of maize seedlings. The expression of all examined mtRNAs is enhanced in vascular bundles, particularly in procambium- and xylem-forming cells. Mitochondrial transcript levels correlated highly with cell division activity. For example, in roots, the transcripts are abundant in the dividing cells of the meristem but drop to very low levels in the nondividing cells of the root cap and the meristem quiescent center. By comparison, levels of functional mitochondria, as assessed by rhodamine-123 fluorescence, did not vary greatly among the same group of cells. In shoots, in situ hybridization and blot hybridization revealed differences in the patterns of localization among different mtRNAs. The results indicate that during vegetative growth, mitochondrial gene expression at the transcript level is subject to an unexpected degree of cell-specific regulation and that different controls may operate on different trancripts.

Regulation of gene expression in plant mitochondria.

Many genes is plant mitochondria have been analyzed in the past 15 years and regulatory processes controlling gene expression can now be investigated. In vitro systems capable of initiating transcription faithfully at promoter sites have been developed for both monocot and dicot plants and will allow the identification of the interacting nucleic acid elements and proteins which specify and guide transcriptional activities. Mitochondrial activity, although required in all plant tissues, is capable of adapting to specific requirements by regulated gene expression. Investigation of the factors governing the quality and quantity of distinct RNAs will define the extent of interorganelle regulatory interference in mitochondrial gene expression.

RNA editing in wheat mitochondria results in the conservation of protein sequences

RNA editing is a process that results in the production of a messenger RNA with nucleotide sequences that differ from those of the template DNA, and provides another mechanism for modulating gene expression. The phenomenon was initially described in the mitochondria of protozoa. Here we report that RNA editing is also required for the correct expression of plant mitochondrial genes. It has previously been proposed that in plant mitochondria there is a departure from the universal genetic code, with CGG specifying tryptophan instead of arginine. This was because CGG codons are often found in plant mitochondrial genes at positions corresponding to those encoding conserved tryptophans in other organisms. We have now found, however, wheat mitochondrial gene sequences containing C residues that are edited to U residues in the corresponding mRNA sequences. In this way, CGG codons can be changed to UGG codons in the mRNA so that tryptophan may be encoded according to the universal genetic code. Furthermore, for each codon modification resulting from a C----U conversion that we studied, we found a corresponding change in the amino acid that was encoded. RNA editing in wheat mitochondria can thus maintain genetic information at the RNA level and as a result contribute to the conservation of mitochondrial protein sequences among plants.