Yeast Mitochondrial Transcription Factor Mtf1 Determines the Precision of Promoter-Directed Initiation of RNA Polymerase Rpo41

The Identification of the Mitochondrial DNA Polymerase γ (Mip1) of the Entomopathogenic Fungus Metarhizium brunneum

Replication of the mitochondrial (mt) genome in filamentous fungi is under-studied, and knowledge is based mainly on data from yeasts and higher eukaryotes. In this study, the mitochondrial DNA polymerase γ (Mip1) of the entomopathogenic fungus Metarhizium brunneum is characterized and analyzed with disruption experiments and its in silico interactions with key proteins implicated in mt gene transcription, i.e., mt RNA polymerase Rpo41 and mt transcription factor Mtf1. Disruption of mip1 gene and its partial expression influences cell growth, morphology, germination and stress tolerance. A putative in silico model of Mip1-Rpo41-Mtf1, which is known to be needed for the initiation of replication, was proposed and helped to identify potential amino acid residues of Mip1 that interact with the Rpo41-Mtf1 complex. Moreover, the reduced expression of mip1 indicates that Mip1 is not required for efficient transcription but only for replication. Functional differences between the M. brunneum Mip1 and its counterparts from Saccharomyces cerevisiae and higher eukaryotes are discussed.

The Pet127 protein is a mitochondrial 5'-to-3' exoribonuclease from the PD-(D/E)XK superfamily involved in RNA maturation and intron degradation in yeasts

Pet127 is a mitochondrial protein found in multiple eukaryotic lineages, but absent from several taxa, including plants and animals. Distant homology suggests that it belongs to the divergent PD-(D/E)XK superfamily which includes various nucleases and related proteins. Earlier yeast genetics experiments suggest that it plays a nonessential role in RNA degradation and 5' end processing. Our phylogenetic analysis suggests that it is a primordial eukaryotic invention that was retained in diverse groups, and independently lost several times in the evolution of other organisms. We demonstrate for the first time that the fungal Pet127 protein in vitro is a processive 5'-to-3' exoribonuclease capable of digesting various substrates in a sequence nonspecific manner. Mutations in conserved residues essential in the PD-(D/E)XK superfamily active site abolish the activity of Pet127. Deletion of the PET127 gene in the pathogenic yeast Candida albicans results in a moderate increase in the steady-state levels of several transcripts and in accumulation of unspliced precursors and intronic sequences of three introns. Mutations in the active site residues result in a phenotype identical to that of the deletant, confirming that the exoribonuclease activity is related to the physiological role of the Pet127 protein. Pet127 activity is, however, not essential for maintaining the mitochondrial respiratory activity in C. albicans.

Mitochondrial Transcription of Entomopathogenic Fungi Reveals Evolutionary Aspects of Mitogenomes

Entomopathogenic fungi and more specifically genera Beauveria and Metarhizium have been exploited for the biological control of pests. Genome analyses are important to understand better their mode of action and thus, improve their efficacy against their hosts. Until now, the sequences of their mitochondrial genomes were studied, but not at the level of transcription. Except of yeasts and Neurospora crassa, whose mt gene transcription is well described, in all other Ascomycota, i.e., Pezizomycotina, related information is extremely scarce. In this work, mt transcription and key enzymes of this function were studied. RT-PCR experiments and Northern hybridizations reveal the transcriptional map of the mt genomes of B. bassiana and M. brunneum species. The mt genes are transcribed in six main transcripts and undergo post-transcriptional modifications to create single gene transcripts. Promoters were determined in both mt genomes with a comparative in silico analysis, including all known information from other fungal mt genomes. The promoter consensus sequence is 5'-ATAGTTATTAT-3' which is in accordance with the definition of the polycistronic transcripts determined with the experiments described above. Moreover, 5'-RACE experiments in the case of premature polycistronic transcript nad1-nad4-atp8-atp6 revealed the 5' end of the RNA transcript immediately after the in silico determined promoter, as also found in other fungal species. Since several conserved elements were retrieved from these analyses compared to the already known data from yeasts and N. crassa, the phylogenetic analyses of mt RNA polymerase (Rpo41) and its transcriptional factor (Mtf1) were performed in order to define their evolution. As expected, it was found that fungal Rpo41 originate from the respective polymerase of T7/T3 phages, while the ancestor of Mtf1 is of alpha-proteobacterial origin. Therefore, this study presents insights about the fidelity of the mt single-subunit phage-like RNA polymerase during transcription, since the correct identification of mt promoters from Rpo41 requires an ortholog to bacterial sigma factor, i.e., Mtf1. Thus, a previously proposed hypothesis of a phage infected alpha-proteobacterium as the endosymbiotic progenitor of mitochondrion is confirmed in this study and further upgraded by the co-evolution of the bacterial (Mtf1) and viral (Rpo41) originated components in one functional unit.

Pentatricopeptide Motifs in the N-Terminal Extension Domain of Yeast Mitochondrial RNA Polymerase Rpo41p Are Not Essential for Its Function

The core mitochondrial RNA polymerase is a single-subunit enzyme that in yeast Saccharomyces cerevisiae is encoded by the nuclear RPO41 gene. It is an evolutionary descendant of the bacteriophage RNA polymerases, but it includes an additional unconserved N-terminal extension (NTE) domain that is unique to the organellar enzymes. This domain mediates interactions between the polymerase and accessory regulatory factors, such as yeast Sls1p and Nam1p. Previous studies demonstrated that deletion of the entire NTE domain results only in a temperature-dependent respiratory deficiency. Several sequences related to the pentatricopeptide (PPR) motifs were identified in silico in Rpo41p, three of which are located in the NTE domain. PPR repeat proteins are a large family of organellar RNA-binding factors, mostly involved in posttranscriptional gene expression mechanisms. To study their function, we analyzed the phenotype of strains bearing Rpo41p variants where each of these motifs was deleted. We found that deletion of any of the three PPR motifs in the NTE domain does not affect respiratory growth at normal temperature, and it results in a moderate decrease in mtDNA stability. Steady-state levels of COX1 and COX2 mRNAs are also moderately affected. Only the deletion of the second motif results in a partial respiratory deficiency, manifested only at elevated temperature. Our results thus indicate that the PPR motifs do not play an essential role in the function of the NTE domain of the mitochondrial RNA polymerase.

High yields and soluble expression of superoxide dismutases in Escherichia coli due to the HIV-1 Tat peptide via increases in mRNA transcription

This study aimed to validate the high yield and soluble expression of proteins carrying the transactivator of transcription (Tat) peptide tag, and further explored the potential mechanism by which the Tat tag increases expression. Escherichia coli superoxide dismutase (SOD) proteins, including SodA, SodB and SodC, were selected for analysis. As expected, the yields and the solubility of Tat-tagged proteins were higher than those of Tat-free proteins, and similar results were observed for the total SOD enzyme activity. Bacterial cells that overexpressed Tat-tagged proteins exhibited increased anti-paraquat activity compared with those expressing Tat-free proteins that manifested as SodA>SodC>SodB. When compared with an MG1655 wild-type strain, the growth of a ΔSodA mutant strain was found to be inhibited after paraquat treatment; the growth of ΔSodB and ΔSodC mutant strains was also slightly inhibited. The mRNA transcript level of genes encoding Tat-tagged proteins was higher than that of genes encoding Tat-free proteins. Furthermore, the α-helix and turn of Tat-tagged proteins were higher than those of Tat-free proteins, but the β-sheet and random coil content was lower. These results indicated that the incorporation of the Tat core peptide as a significant basic membrane transduction peptide in fusion proteins could increase mRNA transcripts and promote the high yield and soluble expression of heterologous proteins in E. coli.

Orphan proteins of unknown function in the mitochondrial intermembrane space proteome: New pathways and metabolic cross-talk

The mitochondrial intermembrane space (IMS) is involved in protein transport, lipid homeostasis and metal ion exchange, while further acting in signalling pathways such as apoptosis. Regulation of these processes involves protein modifications, as well as stress-induced import or release of proteins and other signalling molecules. Even though the IMS is the smallest sub-compartment of mitochondria, its redox state seems to be tightly regulated. However, the way in which this compartment participates in the cross-talk between the multiple organelles and the cytosol is far from understood. Here we focus on newly identified IMS proteins that may represent future challenges in mitochondrial research. We present an overview of the import pathways, the recently discovered new components of the IMS proteome and how these relate to key aspects of cell signalling and progress made in stem cell and cancer research.

•

A brief overview of the classic mitochondrial import pathways is featured

•

Recent studies assigning a number of new proteins to the mitochondrial IMS are discussed

•

Analysis of the expanded IMS proteomes can provide insights into organelle cross-talk and signalling pathways

The Yeast Mitochondrial RNA Polymerase and Transcription Factor Complex Catalyzes Efficient Priming of DNA Synthesis on Single-stranded DNA

Primases use single-stranded (ss) DNAs as templates to synthesize short oligoribonucleotide primers that initiate lagging strand DNA synthesis or reprime DNA synthesis after replication fork collapse, but the origin of this activity in the mitochondria remains unclear. Herein, we show that the Saccharomyces cerevisiae mitochondrial RNA polymerase (Rpo41) and its transcription factor (Mtf1) is an efficient primase that initiates DNA synthesis on ssDNA coated with the yeast mitochondrial ssDNA-binding protein, Rim1. Both Rpo41 and Rpo41-Mtf1 can synthesize short and long RNAs on ssDNA template and prime DNA synthesis by the yeast mitochondrial DNA polymerase Mip1. However, the ssDNA-binding protein Rim1 severely inhibits the RNA synthesis activity of Rpo41, but not the Rpo41-Mtf1 complex, which continues to prime DNA synthesis efficiently in the presence of Rim1. We show that RNAs as short as 10-12 nt serve as primers for DNA synthesis. Characterization of the RNA-DNA products shows that Rpo41 and Rpo41-Mtf1 have slightly different priming specificity. However, both prefer to initiate with ATP from short priming sequences such as 3'-TCC, TTC, and TTT, and the consensus sequence is 3'-Pu(Py)2-3 Based on our studies, we propose that Rpo41-Mtf1 is an attractive candidate for serving as the primase to initiate lagging strand DNA synthesis during normal replication and/or to restart stalled replication from downstream ssDNA.