Mitochondrial genome in animal cells

In the past decade, the development of new DNA, RNA, and protein technologies has greatly incremented the knowledge about the organization and expression of mitochondrial DNA. The complete base sequence of mitochondrial DNA of several animals is known and many data are rapidly accumulating on the mitochondrial genomes of other systems. Here we discuss the results so far obtained that disclosed unexpected features of mitochondrial genetics. Furthermore, mitochondrial DNA has become established as a powerful tool for evolutionary studies in animals. Evidences are preented demonstrating that the evolution of mitochondrial DNA has proceeded in different ways in the various taxonomic groups. Data on heteroplasmic animals, which demonstrate the rapid evolution of mitochondrial DNA, are also presented.     

Assignment of the human heterogeneous nuclear ribonucleoprotein A1 gene (HNRPA1) to chromosome 12q13.1 by cDNA competitive in situ hybridization.

Heterogeneous nuclear ribonucleoprotein (HNRP) core protein A1 is a major component of mammalian HNRP particles. The human HNRP A1 protein was shown to be encoded by a 4.6-kb gene, split into 10 exons, belonging to a multigene family of about 30 A1-specific sequences per haploid genome, many of which correspond to pseudogenes of the processed type. Here we report the mapping of the human HNRPA1 gene to band 12q13.1. Localization was performed by nonisotopic in situ hybridization using a phage genomic clone that contains the active HNRPA1 gene as well as 13.5-kb flanking sequences. To suppress hybridization to pseudogene sequences, unlabeled HNRPA1 cDNA was added in excess over the probe to the hybridization mixture.     

Evolutionary Origin of Nonuniversal Cug(ser) Codon in Some Candida Species as Inferred from a Molecular Phylogeny

CUG, a universal leucine codon, has been reported to be read as serine in various yeast species belonging to the genus Candida. To gain a deeper insight into the origin of this deviation from the universal genetic code, we carried out a phylogenetic analysis based on the small-subunit ribosomal RNA genes from some Candida and other related Hemiascomycetes. Furthermore, we determined the phylogenetic relationships between the tRNA(Ser)CAG, responsible for the translation of CUG, from some Candida species and the other serine and leucine isoacceptor tRNAs in C. cylindracea. We demonstrate that the group of Candida showing the genetic code deviation is monophiletic and that this deviation could have been originated more than 150 million years ago. We also describe how phylogenetic analysis can be used for genetic code predictions.     

Localization of DNA Sequences Tightly Associated with the Synaptonemal Complex in Compositional Fractions of the Golden Hamster Genome*

Synaptonemal complex (SC) isolated from spermatocyte nuclei after their exhaustive hydrolysis by DNase II contains DNA sequences tightly associated with it (SCAR DNA). Here, the compositional properties of a cloned family of golden hamster SCAR DNA were studied. For this purpose, 27 SCAR DNA clones were hybridized with compositionally fractionated golden hamster genomic DNA. The sequences of the SCAR DNA family were mainly localized in the GC-poor isochore families L1 and L2, which accounted for 63% of hybridization signals. The remaining 37% of signals pertained to the GC-rich isochore families H1 and H2. Thus, SCAR DNA proved to be distributed throughout the genome, irrespective of differences in density and sequence type between isochore families. Moreover, the SCAR DNA sequences containing the regions of homology with LINE/SINE repeats were found in all the isochore families. The compositional localization of SCAR DNA is in agreement with the hypothesis that the SC and SCAR DNA participate in chromatin reorganization during meiosis prophase I, which should result in the attachment of chromatin loops to the lateral elements of SC throughout its length.     

Lineage-Specific Evolution of Echinoderm Mitochondrial ATP Synthase Subunit 8

Peculiar evolutionary properties of the subunit 8 of mitochondrial ATP synthase (ATPase8) are revealed by comparative analyses carried out between both closely and distantly related species of echinoderms. The analysis of nucleotide substitution in the three echinoids demonstrated a relaxation of amino acid functional constraints. The deduced protein sequences display a well conserved domain at the N-terminus, while the central part is very variable. At the C-terminus, the broad distribution of positively charged amino acids, which is typical of other organisms, is not conserved in the two different echinoderm classes of the sea urchins and of the sea stars. Instead, a motif of three amino acids, so far not described elsewhere, is conserved in sea urchins and is found to be very similar to the motif present in the sea stars. Our results indicate that the N-terminal region seems to follow the same evolutionary pattern in different organisms, while the maintenance of the C-terminal part in a phylum-specific manner may reflect the co-evolution of mitochondrial and nuclear genes.