Mitochondrial mRNA processing

Abstract: Mitochondrial mRNAs in yeast arise by processing of polygenic primary transcripts at a conserved dodecamer sequence (5'-AAUAAPyAUUCUU-3'). Previous results indicated that processing at dodecamer sites interrupted the sequence implying that it functioned primarily as a signal for 3' end formation of mRNAs. We have determined the precise cleavage site for RNAs processed at the dodecamer sequences associated with the oli1 gene and the omega intron of the 21S rRNA gene. In both cases cleavage occurred two bases downstream of the site. Hydrolysis left the PO4 group attached to the 3' terminus of the cleavage products. These results demonstrate for the first time that mature mitochondrial mRNAs terminate with an intact dodecamer sequence. In light of the recent identification of a protein complex within mitochondria that binds to RNAs terminating with an intact dodecamer sequence, these results support the idea that the dodecamer sequence functions not only within pre-mRNAs as a processing site, but within mature mRNAs as well, possibly for the stabilization and/or translation.

Abstract: The cytochrome b gene in Saccharomyces cerevisiae, COB , is encoded by the mitochondrial genome. Nuclear-encoded Cbp1 protein is required specifically for COB mRNA stabilization. Cbp1 interacts with a CCG element in a 64-nucleotide sequence in the 5′-untranslated region of COB mRNA. Mutation of any nucleotide in the CCG causes the same phenotype as cbp1 mutations, i.e. , destabilization of both COB precursor and mature message. In this study, eleven nuclear suppressors of single-nucleotide mutations in CCG were isolated and characterized. One dominant suppressor is in CBP1 , while the other 10 semidominant suppressors define five distinct linkage groups. One group of four mutations is in PET127 , which is required for 5′ end processing of several mitochondrial mRNAs. Another mutation is linked to DSS1 , which is a subunit of mitochondrial 3′ → 5′ exoribonuclease. A mutation linked to the SOC1 gene, previously defined by recessive mutations that suppress cbp1 ts alleles and stabilize many mitochondrial mRNAs, was also isolated. We hypothesize that the products of the two uncharacterized genes also affect mitochondrial RNA turnover.

Abstract: The gene region coding for subunits α and 9 of the mitochondrial ATP synthase exhibit an identical DNA sequence in wheat, rye, and the intergeneric hybrid triticale (xTriticosecale Wittmack) However, co-transcripts containing both genes show different sizes depending on the nuclear genotype To investigate nuclear-mitochondrial interactions leading to this variation, we performed a comparative transcript analysis with various lines carrying defined nuclear and cytoplasmic genotypes Northern analyses showed that all wheat lines investigated possess a single atpA/atp9 mRNA of 26 kb, whereas in rye and five independent triticale lines an additional transcript of 235 kb appeared Primer-extension and RNase-protection analyses indicate that the co-transcripts of this gene have staggered 5′ termini in some lines, whereas the 3′ termini seem to be similar in wheat, rye, and triticale Transcription is initiated at position-338/-339 upstream of the atpA gene in all lines investigated, giving rise to a 26-kb mRNA In rye and triticale, staggered 5′ termini were observed closer to the translational start The DNA sequences upstream of these termini exhibit homology to plant mitochondrial-processing sites, therefore the proximal 5′ ends are most probably generated by RNA processing As the processing event occurs more frequently in triticale carrying the Triticum timopheevi cytoplasm, trans-acting factors from rye are likely to interact with other cytoplasmic factors resulting in the observed RNA modification Most interestingly, the T timopheevi cytoplasm inducing male sterility in alloplasmic wheat, fails to generate the CMS phenotype in triticale The data support our hypothesis that nuclear factors affect mitochondrial gene expression and thus control sexual fertility in wheat and triticale

Abstract: Although mechanisms that activate organogenesis in plants are well established, much less is known about the local fine-tuning of cell proliferation, crucial for creating well-structured and sized organs. Here we show, through analysis of three temperature-dependent fasciation (TDF) mutants of Arabidopsis, root redifferentiation defective 1 (rrd1), rrd2, and root initiation defective 4 (rid4), that proper mitochondrial RNA processing is required for restrictive control of cell division during lateral root (LR) organogenesis. These mutants form abnormally broadened (i.e. fasciated) LRs under high-temperature conditions due to excessive cell division. We identified RRD1 as encoding a poly(A)-specific ribonuclease (PARN)-like protein and RRD2 and RID4 as encoding pentatricopeptide repeat (PPR) proteins. All TDF proteins were found to be involved in mitochondrial mRNA processing. Phenocopy of TDF mutants by respiratory inhibitors further corroborated our discovery. Our data provide novel insights into the molecular machinery and physiological and developmental roles of mitochondrial mRNA metabolism in plants.