We undertook the characterization of an actin gene and its proximal promoter in the oyster Crassostrea gigas. A complete actin cDNA was identified, sequenced and its amino acid sequence deduced. Comparative analysis showed a high homology with actin of other species and that this gene is closer to the cytoplasmic form of actins than to the muscle type. A probe derived from the 5P P-untranslated region of the cDNA was then used to isolate the actin gene from a genomic library. The gene was sequenced and shown to contain a single 643 bp intron. A 1670 bp fragment upstream from the open reading frame was isolated and sequenced. This upstream region displays typical features of actins such as a serum response element (CarG box). This fragment was cloned into the promoterless vector pGL3-basic and the resulting construct was transfected into cells of dissociated oyster heart primary cultures. Its capacity to express the luciferase in this in vitro homologous system was monitored and showed high expression levels. This is the first complete actin sequence reported so far for the oyster C. gigas and its promoter is the first available among bivalves.z 1999 Federation of European Biochemical Societies.

Endonuclease G (Endo G) is a nuclease of prokaryotic lineage found in the mitochondria of vertebrates that has been suggested to play a role in mitochondrial DNA (mtDNA) replication. We have isolated and sequenced the entire mouse endo G gene, determined the limits of the mRNA, and mapped the promoter region. The coding sequence of the single copy gene is interrupted by two introns and analysis of the transcripts does not support a model by which more than one Endo G isoform could be produced by alternative splicing. We have also characterized a full-length human Endo G cDNA and comparison at the protein level of the human, bovine, and murine nucleases indicates a high degree of conservation except in the respective mitochondrial targeting signals. Endo G is ubiquitously expressed and the steady-state levels of its mRNA vary by a factor greater than seven between different tissues. The relationship between the mtDNA copy number and Endo G mRNA levels is not strictly proportional but tissues richer in mtDNA have higher amounts of the mRNA and vice versa.

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