DNA IN MITOCHONDRIA OF NEUROSPORA CRASSA

Non-Coding RNAs of Mitochondrial Origin: Roles in Cell Division and Implications in Cancer

Non-coding RNAs (ncRNAs) are a heterogeneous group, in terms of structure and sequence length, consisting of RNA molecules that do not code for proteins. These ncRNAs have a central role in the regulation of gene expression and are virtually involved in every process analyzed, ensuring cellular homeostasis. Although, over the years, much research has focused on the characterization of non-coding transcripts of nuclear origin, improved bioinformatic tools and next-generation sequencing (NGS) platforms have allowed the identification of hundreds of ncRNAs transcribed from the mitochondrial genome (mt-ncRNA), including long non-coding RNA (lncRNA), circular RNA (circRNA), and microRNA (miR). Mt-ncRNAs have been described in diverse cellular processes such as mitochondrial proteome homeostasis and retrograde signaling; however, the function of the majority of mt-ncRNAs remains unknown. This review focuses on a subgroup of human mt-ncRNAs whose dysfunction is associated with both failures in cell cycle regulation, leading to defects in cell growth, cell proliferation, and apoptosis, and the development of tumor hallmarks, such as cell migration and metastasis formation, thus contributing to carcinogenesis and tumor development. Here we provide an overview of the mt-ncRNAs/cancer relationship that could help the future development of new biomedical applications in the field of oncology.

Evolutionary genetics of the mitochondrial genome: insights from Drosophila

Mitochondria are key to energy conversion in virtually all eukaryotes. Intriguingly, despite billions of years of evolution inside the eukaryote, mitochondria have retained their own small set of genes involved in the regulation of oxidative phosphorylation (OXPHOS) and protein translation. Although there was a long-standing assumption that the genetic variation found within the mitochondria would be selectively neutral, research over the past 3 decades has challenged this assumption. This research has provided novel insight into the genetic and evolutionary forces that shape mitochondrial evolution and broader implications for evolutionary ecological processes. Many of the seminal studies in this field, from the inception of the research field to current studies, have been conducted using Drosophila flies, thus establishing the species as a model system for studies in mitochondrial evolutionary biology. In this review, we comprehensively review these studies, from those focusing on genetic processes shaping evolution within the mitochondrial genome, to those examining the evolutionary implications of interactions between genes spanning mitochondrial and nuclear genomes, and to those investigating the dynamics of mitochondrial heteroplasmy. We synthesize the contribution of these studies to shaping our understanding of the evolutionary and ecological implications of mitochondrial genetic variation.

The Interplay between Dysregulated Metabolism and Epigenetics in Cancer

Cellular metabolism (or energetics) and epigenetics are tightly coupled cellular processes. It is arguable that of all the described cancer hallmarks, dysregulated cellular energetics and epigenetics are the most tightly coregulated. Cellular metabolic states regulate and drive epigenetic changes while also being capable of influencing, if not driving, epigenetic reprogramming. Conversely, epigenetic changes can drive altered and compensatory metabolic states. Cancer cells meticulously modify and control each of these two linked cellular processes in order to maintain their tumorigenic potential and capacity. This review aims to explore the interplay between these two processes and discuss how each affects the other, driving and enhancing tumorigenic states in certain contexts.

From TOM to the TIM23 complex - handing over of a precursor

Mitochondrial precursor proteins with amino-terminal presequences are imported via the presequence pathway, utilizing the TIM23 complex for inner membrane translocation. Initially, the precursors pass the outer membrane through the TOM complex and are handed over to the TIM23 complex where they are sorted into the inner membrane or translocated into the matrix. This handover process depends on the receptor proteins at the inner membrane, Tim50 and Tim23, which are critical for efficient import. In this review, we summarize key findings that shaped the current concepts of protein translocation along the presequence import pathway, with a particular focus on the precursor handover process from TOM to the TIM23 complex. In addition, we discuss functions of the human TIM23 pathway and the recently uncovered pathogenic mutations in TIM50.

Sunlight radiation as a villain and hero: 60 years of illuminating research

In the 60 years since the inaugural edition of the International Journal of Radiation Biology, much of our understanding of the biological effects of solar radiation has changed. Earlier in the century, sunlight played a 'hero's' role in reducing disabling rickets, while today debate still continues on the amount of sun required before exposure reveals the 'villainous' side of solar radiation. Although knowledge of the ultra violet (UV) component of sunlight as a carcinogen has become widespread, skin cancer rates are still rising yearly. Twentieth century attitudes have seen an about-face in the field of dermatological sun protection, with sunscreens changing from recipes designed to promote a 'healthy tan' to formulations proven to block both ultraviolet B (UVB) and more recently, ultraviolet A (UVA), to minimize premature sun-aging and skin cancer risk. In the early 1960s, DNA was first found to exist within mitochondria, while recently the connections between mitochondrial changes and UV radiation exposure have been expanded. Sixty years ago, understanding of the endocrine systems of mammals was enjoying its infancy. Early discoveries that light, particularly natural light, could have profound effects on functions such as sleep patterns and hormonal balance were made, while today more advanced knowledge has led to lighting improvements having pronounced effects on human wellbeing. Photosensitization 60 years ago was a health concern for both humans and their domestic animals, while today chemically engineered photosensitizing drugs can be administered along with highly directed light to pinpoint delivery targets for drug action. Life on earth is inextricably bound up with solar radiation. This article attempts to outline many of the ways in which our opinions about solar radiation have changed since the journal's inception.

Energy, ageing, fidelity and sex: oocyte mitochondrial DNA as a protected genetic template

Oxidative phosphorylation couples ATP synthesis to respiratory electron transport. In eukaryotes, this coupling occurs in mitochondria, which carry DNA. Respiratory electron transport in the presence of molecular oxygen generates free radicals, reactive oxygen species (ROS), which are mutagenic. In animals, mutational damage to mitochondrial DNA therefore accumulates within the lifespan of the individual. Fertilization generally requires motility of one gamete, and motility requires ATP. It has been proposed that oxidative phosphorylation is nevertheless absent in the special case of quiescent, template mitochondria, that these remain sequestered in oocytes and female germ lines and that oocyte mitochondrial DNA is thus protected from damage, but evidence to support that view has hitherto been lacking. Here we show that female gametes of Aurelia aurita, the common jellyfish, do not transcribe mitochondrial DNA, lack electron transport, and produce no free radicals. In contrast, male gametes actively transcribe mitochondrial genes for respiratory chain components and produce ROS. Electron microscopy shows that this functional division of labour between sperm and egg is accompanied by contrasting mitochondrial morphology. We suggest that mitochondrial anisogamy underlies division of any animal species into two sexes with complementary roles in sexual reproduction. We predict that quiescent oocyte mitochondria contain DNA as an unexpressed template that avoids mutational accumulation by being transmitted through the female germ line. The active descendants of oocyte mitochondria perform oxidative phosphorylation in somatic cells and in male gametes of each new generation, and the mutations that they accumulated are not inherited. We propose that the avoidance of ROS-dependent mutation is the evolutionary pressure underlying maternal mitochondrial inheritance and the developmental origin of the female germ line.

mtDNA makes a U-turn for the mitochondrial nucleoid

Mitochondria contain mtDNA derived from the ancestral endosymbiont genome. Important subunits of the oxidative phosphorylation system, which supplies cells with the energy currency ATP, are encoded by mtDNA. A naked mtDNA molecule is longer than a typical mitochondrion and is therefore compacted in vivo to form a nucleoprotein complex, denoted the mitochondrial nucleoid. Mitochondrial transcription factor A (TFAM) is the main factor packaging mtDNA into nucleoids and is also essential for mtDNA transcription initiation. The crystal structure of TFAM shows that it bends mtDNA in a sharp U-turn, which likely provides the structural basis for its dual functions. Super-resolution imaging studies have revealed that the nucleoid has an average diameter of ∼100nm and frequently contains a single copy of mtDNA. In this review the structure of the mitochondrial nucleoid and its possible regulatory roles in mtDNA expression will be discussed.

XV.—The metabolism of DNA in the liver during precocious induction of metamorphosis in Rana catesbeiana

Studies have been made on the incorporation of [8H]-deoxythymidine into the DNA of the livers of Rana catesbeiana tadpoles. When metamorphosis was induced with tri-iodothyronine, the specific activity of the nuclear DNA rose 5 days after administration of the hormone. In contrast, the specific activity of the DNA from the mitochondrial fraction rose grave–2 days after hormone administration.

In order to determine whether the in vivo change was due to alterations in the pool sizes of the DNA precursors, in vitro studies on DNA polymerase were carried out. It was found that under conditions where the enzyme activity was not limited by availability of template or substrates, there was a rise in the DNA polymerase activity in crude cell extracts from the tadpole liver. Fractionation of the cell components showed that little of this increment in activity appeared to be located in the nucleus, but that a large percentage alteration in activity occurred in the mitochondrial and cell sap fractions.

A possible interpretation of these results is that an increase in the mitochondrial DNA polymerase is one of the early effects of thyroid hormone. This possibility is discussed in relation to the other known effects of thyroid hormones in tadpoles, with particular reference to nucleic acid metabolism and also to mitochondrial hyperplasia.

Observations on iron uptake, iron metabolism, cytochrome c content, cytochrome a content and cytochrome c-oxidase activity in regenerating rat liver

1. Differential and density-gradient centrifugation were used to fractionate mitochondria and fluffy layer from normal and regenerating rat liver. The iron, cytochrome a and cytochrome c contents and cytochrome c-oxidase activity were studied as well as the uptake of (59)Fe into protein and cytochrome c. 2. A certain degree of heterogeneity was evident between the heavy-mitochondrial and light-mitochondrial fractions, and in their behaviour during liver regeneration. 3. The specific content of light-mitochondrial iron and cytochrome a was 1.3-1.4 times that of heavy mitochondria. Changes in cytochrome c-oxidase activity closely followed those of cytochrome a content during liver regeneration, but not for light mitochondria after 10 days. 4. Radioactive iron ((59)Fe) was most actively taken up by well-washed light mitochondria during early liver regeneration. After 22 days fluffy layer became preferentially labelled. This substantiates the view that fluffy layer partially represents broken-down mitochondria. 5. During early regeneration, light-mitochondrial fractions separated along a density gradient were about 3 times as radioactive, and showed distinct heterogeneity of (59)Fe-labelling, in contrast with near homogeneity for heavy mitochondria. 6. Immediately after partial hepatectomy fractions corresponding to density 1.155 were 5-10 times as radioactive as particles of greater density. The radioactivity decreased sharply after 6 days. 7. These particles of low density possessed higher NADH-cytochrome c-reductase (1.5-5-fold) and succinate-dehydrogenase (1.1-2-fold) activities than typical mitochondrial fractions. Their succinate-cytochrome c-reductase and cytochrome c-oxidase activities were slightly lower. 8. The results are discussed in relation to mitochondrial morphogenesis, and a possible route from submitochondrial particles is suggested.

Role of Mitochondrial DNA in the Senescence and Hypovirulence of Fungi and Potential for Plant Disease Control

The unique coenocytic anatomy of the mycelia of the filamentous fungi and the formation of anastomoses between hyphae from different mycelia enable the intracellular accumulation and infectious transmission of plasmids and mutant mitochondrial DNAs (mtDNAs) that cause senescence. For reasons that are not fully apparent, mitochondria that are rendered dysfunctional by so-called "suppressive" mtDNA mutations proliferate rapidly in growing cells and gradually displace organelles that contain wild-type mtDNA molecules and are functional. The consequence of this process is senescence and death if the suppressive mtDNA contains a lethal mutation. Suppressive mtDNA mutations and mitochondrial plasmids can elicit cytoplasmically transmissible "mitochondrial hypovirulence" syndromes in at least some of the phytopathogenic fungi. In the chestnut-blight fungus Cryphonectria parasitica, the pattern of asexual transmission of mutant mtDNAs and mitochondrial plasmids resembles the pattern of "infectious" transmission displayed by the attenuating virus that is most commonly used for the biological control of this fungus. At least some of the attenuating mitochondrial hypovirulence factors are inherited maternally in crosses, whereas the viruses are not transmitted sexually. The natural control of blight in an isolated stand of chestnut trees has resulted from the invasion of the local population of C. parasitica by a senescence-inducing mutant mtDNA. Moreover, a mitochondrial plasmid, pCRY1, attenuates at least some virulent strains of C. parasitica, suggesting that such factors could be applied to control plant diseases caused by fungi.

Transport of newly synthesized proteins into mitochondria - a review

This review examines the mechanism of translocation of cytoplasmically synthesized proteins into mitochondria. Approximately 10% of the mitochondrial proteins are synthesized within the organelles while most mitochondrial proteins are coded for by nuclear genes and synthesized on cytoplasmic ribosomes. Those mitochondrial proteins synthesized on cytoplasmic ribosomes have to be transferred at some point into one of the mitochondrial compartments, a process which would require their insertion through one or both mitochondrial membranes. Data accumulated during the past five years indicate that the cytoplasmically synthesized mitochondrial proteins are synthesized on free polysomes then released into the cytoplasm. Most of the proteins examined so far are synthesized in the cytoplasm as larger precursors whose conformations may differ from the conformations of their respective mature forms. These precursor proteins become translocated into mitochondrial post-translationally and processed to their mature forms either during or immediately following translocation into the organelles. The translocation step appears to require mitochondrial ATP. Some processing activities have been localized in the matrix fractions of mitochondria from liver and yeast and they appear to be associated with soluble endopeptidases which act selectively on precursors of mitochondrial proteins. Although it is not clear how the precursor proteins interact with or recognize mitochondrial membranes, studies in yeast indicate that the interactions occur at specific regions on the other mitochondrial membranes.

Studies on phospholipase activities in Neurospora crassa mycelia

Phospholipase A activity has been detected in mycelial homogenate of Neurospora crassa. A submycelial fraction, obtained by differential centrifugation containing the highest specific activity of phospholipase A has been shown to contain ca. 66% phospholipase A and 34% phospholipase A activity along with lysophospholipase and degergent-stimulated phospholipase D activity. Phospholipase A activity bound to N. crassa mycelia also has been observed.

Mitochondrial DNA from Podospora anserina. I. Isolation and characterization

Mitochondrial (Mt) DNA from Podospora anserina was isolated and characterized with respect to density in CsCl, contour length and endonuclease restriction enzymes. The density of Mt DNA for four races examined was 1.694 g/cm3, compared with 1.712 g/cm3 for nuclear DNA. Extraction in the presence of a nuclease inhibitor, aurintricarboxylic acid and isolation in DAPI CsCl gradients allowed us to isolate high molecular weight DNA. Mt DNA isolated by total DNA extraction contained ca. 1% of circular molecules, 31 micron in contour length; Mt DNA isolated from purified mitochondria contained 2--4% of these 31 micron circles. Analysis with Eco RI restriction endonuclease revealed that each of the four races examined, s, A, T and E had a characteristic fragment pattern. Races s and A Mt DNA differed by only one fragment after Eco RI enzymatic digestion; similarly, these two DNA differed by only one or two fragments after Hae III digestion.

A regulatory system controlling the production of cytochrome aa3 in Neurospora crassa

The mitochondria of the cyt-2-1, cya-3-16, cya-4-23 and 299-1 nuclear mutants and the [mi-3] and [exn-5] cytoplasmic mutants of Neurospora crassa are deficient in cytochrome aa3, while the cyb-1-1 and cyb-2-1 mutants have mitochondrial b-cytochrome deficiencies. However, the mitochondria from cyb-1-1 cyt-2-1, cyb-1-1 [mi-3] and cyb-2-1 [mi-3] double mutants contain 30% to 50% of the amount of cytochrome aa3 that is present in mitochondria from wild-type; i.e. cyb-1-1 and cyb-2-2 act as suppressors of the cytochrome aa3 deficiency phenotypes that are associated with cyt-2-1 and [mi-3] mutations. The production of cytochrome aa3 can be induced in cyt-2-1 and [mi-3] by growing cells in medium containing antimycin A, an inhibitor of electron transport in the cytochrome bc1 segment of the mitochondrial electrontransport chain. Moreover, the growth of the [mi-3] mutant is strongly stimulated by low concentrations of antimycin A. The induction of cytochrome aa3 by antimycin treatments does not occur in [exn-5], cya-4-23 and 299-1 cells; but does take place in cya-3-16 cells. Although some of the seven constituent polypeptides of cytochrome aa3 are present in mitochondria of [mi-3], the holoenzyme complex is not formed in the mutant. In contrast, the mitochondria of cyb-1-1 [mi-3] and cyb-2-2 [mi-3] double mutants contain a fully assembled cytochrome oxidase complex as well as some unassembled subunit polypeptides. The observations are indicative of the existence of at least two regulatory systems controlling the production of cytochrome aa3. One of the circuits appears to control the basal or "constitutive" production of cytochrome oxidase, the other seems to coordinate the level of cytochrome aa3 with some function of the mitochondrial cytochrome bc1 complex, possibly electron transport.

Tumorigenicity and its suppression in cybrids of mouse and Chinese hamster cell lines

The effect of cytoplasm upon the expression of tumorigenicity was examined with a pair of mouse and a pair of Chinese hamster cell lines, in intraspecies cybrids formed by reciprocal fusions between either tumorigenic or nontumorigenic cells and cytoplasms derived from them. With the mouse cells, 3T3 and the simian virus 40-transformed line SVT2, the cybrid clones were tumorigenic when SVT2 cells were fused with 3T3 cytoplasts, but not in the reciprocal fusion. With the hamster cells, CHEF/18 and the spontaneous transformant CHEF/16, however, tumorigenicity was partially suppressed in cybrid clones formed by fusion of tumorigenic CHEF/16 cells with CHEF/18 cytoplasts; cybrids were nontumorigenic in the reciprocal fusion. Thus, cybrid analysis has shown that tumorigenicity is not cytoplasmically transmitted in these two cell pairs, but suppression of tumor-forming ability may be cytoplasmically transmitted in the hamster cybrids.

Nuclear suppressors of the (poky) cytoplasmic mutant in Neurospora crassa. II. Mitochondrial cytochrome systems

The mitochondrial cytochrome aa3 and b deficiencies of the [poky] cytoplasmic mutant of Neurospora crassa are partially suppressed by mutant alleles of any one of six nuclear genes, namely sup-1, sup-3, sup-4, sup-5, sup-10 and sup-14. The suppressor-induced increases in the concentration of both cytochromes are detected in the mitochondria from exponentially growing [poky] cultures, and, thus, are clearly distinguishable from the age-dependent changes in the cytochrome system that occur in cultures that approach, or have reached, the stationary phase of growth. The relative amounts of mitochondrial cytochromes aa3 and b show a direct correlation with the relative efficiency of the various sup genes as suppressors of the slow-growth phenotype of [poky]. Since [poky] is defective in mitochondrial protein synthesis due to a lack of 30 S mitochondrial ribosomal subunits, it is proposed that the six suppressors promote the assembly of functional mitochondrial ribosomes.

Mitochondrial genes and cell heredity

It is well known that mitochondria are only partly an autonomous system since they are subjected to nuclear control. For this reason, in studying mitochondrial genes one has to consider constantly the integration of mitochondrial and nuclear genetic systems. This fact makes experimental approaches still more sophisticated, especially, when one turns from individual genetic structures to mitochondrial heredity on the level of cells and multicellular organisms. Here we shall discuss some theoretical aspects of mitochondrial heredity that have been comparatively rarely dealt with in the literature.

Quantitation of mitochondrial DNA, RNA, and protein in starved and starved-refed rat liver

The purpose of this study was to investigate the problem of mitochondrial biogenesis in rat liver. The approach consisted of isolating mitochondria from control, 6 day starved and 6 day starved-5 day refed rats and comparing their DNA, RNA and protein content. This was performed by isolating the mitochondria by reorienting rate zonal centrifugation in sucrose gradients. It was found that six days of starvation resulted in a loss of 30% of the body weight, 55% of the liver weight, 40% of the mitochondrial protein, 60% of the mitochondrial RNA, but only 20% of the mitrochondrial DNA. It was also shown that refeeding of the rats for five days resulted in a restoration to normal or near normal levels in all the parameters measured. Further experiments employing the incorporation of 3H-TTP into into isolated mitochondria indicated that the maintenance of mitochondrial DNA was not the result of continuous DNA sythesis.

Enzymes of deoxythymidine triphosphate biosynthesis in Neurospora crassa mitochondria

Intact mitochondria of Neurospora crassa incorporate deoxythymidine 5'-monophosphate (dTMP) into deoxyribonucleic acid but not the label from (methyl-3H) deoxythymidine. Mitochondrial homogenates contain deoxythymidylate kinase (EC 2.7.4.9), deoxycytidylate aminohydrolase (dCMP deaminase) (EC 3.5.4.12), and thymidylate synthetase (EC 2.1.1b), but not thymidine kinase (EC 2.7.1.21) activity. dTMP kinase is loosely bound to the mitochondrial membrane and is solubilized by 0.4 M KCl in mitochondrial homogenates, the dCMP aminohydrolase deaminase) is bound to the inner membrane and is not solubilized by 0.4 M KCl. dTMP synthetase activity is found in the 2,000 times g particulate fractions by homogenization of mitochondria in 0.4 M KCl. The dCMP deaminase activity found in the particulate fraction of the inner membrane is efficiently regulated by the products of the pathway: deoxycytidine 5'-triphosphate activates whereas deoxythymidine 5'-triphosphate inhibits, as found for the soluble enzyme from other sources. These data indicate that mitochondria of N. crassa contain specific enzymes for the biosynthesis of deoxythymidine triphosphate.

Effects of cycloheximide, D-threo-chloramphenicol, erythromycin and actinomycin D on De-novo synthesis of cytoplasmic and mitochondrial proteins in the cotyledons of germinating pea seeds

Inhibitors of, and radioactive substrates for, protein synthesis were introduced into germinating pea (Pisum sativum L.) seeds, and protein synthesis was allowed to proceed in vivo. Subsequent analyses of subcellular fractions showed the following: Cycloheximide strongly inhibited the incorporation of [(14)C]leucine into both mitochondrial and cytoplasmic proteins. D-Threo-chloramphenicol and erythromycin did not affect cytoplasmic protein synthesis, but partially inhibited mitochondrial protein synthesis. These results suggest that most of the new mitochondrial proteins were originally synthesized in the cytoplasm. Actinomycin D did not appreciably affect the initial incorporation of [(14)C]leucine into either mitochondrial or cytoplasmic proteins, suggesting that information (mRNA) concerning the initially synthesized proteins may be present in the quiescent seeds. The lack of appreciable incorporation of [(3)H]thymidine into mitochondrial DNA supported our previons report that mitochondria may not be synthesized de novo in pea cotyledons.

Direct counting and sizing of mitochondria in solution

Resistive particle counting has been developed for the accurate sizing and counting of mitochondria in solution. The normal detection limit with a 30 micro aperture is 0.48 micro diameter, or 0.056 micro(3) particle volume The mean volume of rat liver mitochondria was 0.42 micro(3) or 0.93 micro in diameter. The average value for numbers of particles per milligram of mitochondrial protein was 4.3 x 10(3), and per gram of rat liver was about 11 x 10(10). These values compare satisfactorily with those derived by light microscopy and electron microscopy. The mean volume for mitochondria from rat heart was 0 60 micro(3) and from rat kidney cortex, 0.23 micro(3). These values agree within 15% of those determined by electron microscopy of whole tissue. Mitochondrial fragility and contaminating subcellular organelles were shown to have little influence on the experimentally determined size distributions The technique may be applied to rapid swelling studies, as well as to estimations of the number and size of mitochondria from animals under different conditions such as liver regeneration and hormonal, pathological, or drug-induced states Mitochondrial DNA, RNA, cytochrome c-oxidase, cytochrome (a / a(3)), and iron were nearly constant per particle over large differences in particle size. Such data may be particularly valuable for biogenesis studies and support the hypothesis that the net amount per particle of certain mitochondrial constituents remains constant during mitochondrial growth and enlargement

Physicochemical characterization of mitochondrial DNA from pea leaves

The mitochondrial DNA from pea leaves exists in a circular conformation. 25% of the circular molecules exist as supercoils, and 10% of the molecules are dimers. The molecular weight of mitochondrial DNA is about 66 to 70 x 10(6) by electron microscopy, and 74 x 10(6) from its renaturation kinetics. No evidence for inter- and intramolecular heterogeneity is found.

The intracellular site of synthesis of mitochndrial ribosomal proteins in Neurospora crassa

The intracellular site of synthesis of mitochondrial ribosomal proteins (MRP) in Neurospora crassa has been investigated using three complementary approaches. (a) Mitochondrial protein synthesis in vitro: Tritium-labeled proteins made by isolated mitochondria were compared to (14)C-labeled marker MRP by cofractionation in a two-step procedure involving isoelectric focusing and polyacrylamide gel electrophoresis. Examination of the electrophoretic profiles showed that essentially none of the peaks of in vitro product corresponded exactly to any of the MRP marker peaks. (b) Sensitivity of in vivo MRP synthesis to chloramphenicol: Cells were labeled with leucine-(3)H in the presence of chloramphenicol, mitochondrial ribosomal subunits were subsequently isolated, and their proteins fractionated by isoelectric focusing followed by gel electrophoresis. The labeling of every single MRP was found to be insensitive to chloramphenicol, a selective inhibitor of mitochondrial protein synthesis. (c) Sensitivity of in vivo MRP synthesis to anisomycin: We have found this antibiotic to be a good selective inhibitor of cytoplasmic protein synthesis in Neurospora. In the presence of anisomycin the labeling of virtually all MRP is inhibited to the same extent as the labeling of cytoplasmic ribosomal proteins. On the basis of these three types of studies we conclude that most if not all 53 structural proteins of mitochondrial ribosomal subunits in Neurospora are synthesized by cytoplasmic ribosomes.