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In rabbit we observed heteroplasmy at an exceptionally high level, the heterogeneity occurring within the noncoding region of the DNA. Mitochondria1 DNA (mt DNA) was cloned in pBR322 and the nucleotide sequence analysis of an EcoRI -Hind I11 fragment encompassing the non-coding region revealed that although there are common features with other mammalian mtDNAs (termed large central-conserved-sequence block, conservedsequence blocks 1, 2 and 3 and termination-associated elements) the non-coding region shows an unusual organization; two stretches of tandem repeats of 20 bp and 1.53 bp are present in a part containing the origin of H-strand replication (Of,) and probably the promoters for transcription as judged from other vertebrates. The long repeats are located between tRNAPhc and conserved sequence block 3 and the short repeats are located between conserved sequence blocks 1 and 2. When cloned in Escherichiu coli (recA or recBC sbcb) DNA fragments containing the short repeats show length differences corresponding to various copy numbers of repeats. Electrophoretic analysis of the appropriate restriction fragments of rabbit mtDNA reveals extended intra-and inter-individual length heterogeneity. Both sets of repeats are involved in the generation of heterogeneity and are present in variable copy numbers from one mtDNA molecule to another. Moreover, rearrangement of the motives of the short repeat are observed to different extents in the mtDNA from one animal to another. The occurrence, maintenance and possible involvement of these repeated sequences, capable of forming stable secondary structures, are discussed in relation to their location in the region of control signals.Mammalian mitochondrial DNAs (mtDNAs) range in size from 16 kb to 17.5 kb. The complete nucleotide sequences of human (Andersonet al., 1981)mouse (Bibbet al., 1981) bovine (Anderson et al., 1982) and rat (Gadaleta et al., 1989) mtDNAs reveal an overall conservation of gene order and very compact organization of genetic information (for reviews see Attardi, 1985, and Tzagoloff, 1985). In all species the noncoding sequences consist of a few short intergenic segments and a large non-coding region located between the genes coding for tRNAPhe and tRNAPr0.The length of this non-coding region varies among species and only the central part exhibits extended nucleotide sequence similarities. This large central-conserved sequence block appears to be one of the mtDNA's most conserved regions, other parts of the non-coding region are the least conserved sequences of the molecule between species. At the level of the species the large conserved sequence block does not diverge anymore than the mitochondrial genes coding for polypeptides (Brown et al., 1986). Several authors have shown that despite sequence diversity these regions share similar physical properties (Mignotte et al., 1987) and potential secondary structures (Brown et al., 1986 andBrun, 1987). The evolutionary pressure on the non-coding region seems to take place at the level of the secondary ...

The effect of the switch to aerobic growth conditions was examined in rabbit articular chondrocytes transferred to culture. Spectroscopic analysis of the cytochromes of the respiratory chain shows that only cytochrome b is present in chondrocytes from cartilage, cytochromes c, c1, and a.a3 being undetectable as compared with the typical spectrum found in a primary cell culture on day 4. Steady state levels of RNA transcripts of nuclear (cytochrome c) and mitochondrial genes (cytochrome b and cytochrome oxidase subunits II and III) involved in the oxidative metabolism were determined relative to the RNA transcripts of the nuclear gene for glyceraldehyde phosphate dehydrogenase involved in the glycolytic pathway and to mitochondrial ribosomal RNAs. Chondrocytes transferred to culture showed a general increase in the levels of all transcripts, but the effect on mitochondrial transcripts was much greater (x 20) than the effect on nuclear transcripts (x 3-4). These results show the absence of a coordinate regulation of the expression of mitochondrial and nuclear genes coding for components of the respiratory chain. The increase in mitochondrial DNA triggered by culture conditions does not appear to be sufficient to account for the enhanced transcription. Concomitant with these mitochondrial changes, the level of transcripts for the collagen II gene involved in the differentiation function decreases dramatically (3% of the control on day 3).

In actively growing vitellogenic oocytes of Xenopus laevis mitochondria segregate into 2 populations. One stays around the nucleus, actively replicates mitochondrial DNA (mtDNA), and builds up most of the stock of the mitochondria in the full-grown oocyte. The other moves toward the vegetal pole and stops replicating mtDNA early in vitellogenesis. Organelles of this population are components of the germ plasm of the cell.

The noti-coding region of rabbit mitochondrial DNA (mtDNA) exhibits two sets of tiindem repeats between conserved sequencc block 1 (CSBI ) and the tRNAP"' gene. Both repeated sequenccs, short repeats (SR) and long repeats (LK), which contain 20 and 153 nucleotides, respectively, urc involvcd in the generation of a high degrce of mitochondrial hetcroplasmy. Due t o the location of these sequences in the rcgulatory region and their properties in terms of variable conformations, they could affect the initiation of replication of the heavy-strand DNA (H-strand DNA) and subsequently would influence the efficiency (if mtDNA replication. The extremities of the displacement loop (I)-loop) DNA strand and thc 5' ends of RNA primers initiating thc H-strand DNA synthesis werc charactcrized in individual rabbits.Mapping at the nucleotide level of the 5' and 3' cnds of the D-loop DNA strands indicates that both cxtremities are heterogeneous. The H-strand replication origin OH is located close 10 the conscrved sequence block CSBl ;is in other mammals. In all of the individuals studicd so far, DNA molecules with it 5' end 1-2 nucleotides downstream of CSBl wcre always present. As H-strand DNA replicution is bclieved to he primed by RNA transcribed frorn the light-strand promoter (LSP), RNA mapping was carried out to identify the 5' end of H-strand RNA. Neighbouring initiation sites were identified at the nucleotide level in an (A+T)-rich region at nucleotirie 1841 and in a stretch of cytosine residues at nuclcotides 1849-1852, which are located at the beginning of the first long repeat. A detailed RNA analysis indicates that H-strand RNA molecules are initiated in each long repeat. The amplification of the rcgulatory region has produced multiple initiation transcription sites and a family of RNA pri tners of various lengths. These variations in length and the ensuing sccondary structures are not critical for their potential function as H-strand DNA replication primers.Keywords: mitochondria; repeats; heteroplasmy ; replication ; trenscription.Tn mammalian cells and probably in all vertebrate cells the replication process of mtDNA is characterized by the asymmetric and asynchronous replication of each strand of DNA with the two origins located far apart on the molecule. The synthesis of the heavy strand (H-strand) stiit-ts first and proceeds along about 60% of the molecule befcm the synthesis of the L-strand can be initiated. In species whcre the origin OH has been precisely mapped, it has been found just downstream of the conserved sequence block CSB 1. Replication at OH is primed by an RNA which in mouse was shown (Chang et al., 1985) to initiate at the promoter for transcription of thc LDNA strand (LSP); therefore, the initiation of transcription at this promoter must be a critical step for the priiiiing of replication as well as for transcription itself (Clayton, 1992). The initiation sites that have C'r,r~c.s/,r,Jic/e~~~~~ to M. CuBride,

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The origin of the mitochondrial mass, previously well characterized in Xenupus diplotene oocytes, has been traced up to'oogonia by means of electron microscopy. A polarized organization of the oogonia and of the oocytes of the succeeding stages was observed. The mitochondrial cloud was found to be built up in the centriolar region near the site where the chromosomes will be implanted along the nuclear envelope at the "bouquet" stage. Autoradiographic studies of thymidine incorporation into mitochondrial DNA suggest that mitochondrial DNA synthesis is active throughout this early period of oogenesis.

In order to clarify some controversial phylogenies such as those regarding the triplet of human, rodent, and cow and the evolutionary position of Lagomorpha with respect to other mammals, we have analyzed both nuclear and mitochondrial genes using the stationary Markov model developed in our laboratory. We found that the two sets of genes give different results. In particular the mitochondrial tree showed rabbit linked first to rodents and the rabbit-rodents branch linked to artiodactyls with human as the outgroup. The most favorite nuclear tree showed human linked first to artiodactyls and the human-artiodactyls branch linked to rabbit with rodents as the outgroup. The obvious questions, (1) which tree is the correct one, or (2) both trees can be incorrect, and (3) how can we explain such an evolutionary pattern, are discussed on the basis of our limited knowledge of factors that influence the clocklike behavior of biological macromolecules.