COMPLETE SEQUENCE OF THE MITOCHONDRIAL-DNA OF THE RHODOPHYTE CHONDRUS-CRISPUS (GIGARTINALES) - GENE CONTENT AND GENOME ORGANIZATION

The complete nucleotide sequence of the circular mitochondrial (mt) DN A from the red alga Chondrus crispus was determined (25,836 nucleotide s, A + T content 72.1%). Fifty one genes were identified. They include genes encoding three subunits of the cytochrome oxidase (cox1 to 3), apocytochrome b (cob), seven subunits of the NADH dehydrogenase comple x (nad1 to 6, nad4L), two ATPase subunits (atp6 and atp9), three ribos omal RNAs (rrn5, srn and lrn), 23 tRNAs and four ribosomal proteins (r ps3, rps11, rps12 and rpl16). Two subunits of the succinate dehydrogen ase complex (sdhB and sdhC), usually found on nuclear genomes, are als o located on the mtDNA of C. crispus. One group IIb intron is inserted in the tRNA(Ile) gene. Six potentially functional open reading frames were identified, four of them having counterparts among green plant m tDNAs. The use of a modified genetic code and the absence of RNA editi ng, previously reported for the cox3 gene, appears as a general charac teristic of this molecule. Mitochondrial genes are encoded on both DNA strands, in two opposite major transcriptional directions, suggesting the existence of two main transcriptional units. Two long and stable stem-loops were identified in intergenic regions, which are believed t o be involved with transcription and replication. The main structural features of this genome are compared with the overall organization of mtDNAs and are discussed in view of the evolution of mitochondria.