COMPLETE DNA-SEQUENCE OF THE MITOCHONDRIAL GENOME OF THE BLACK CHITON, KATHARINA-TUNICATA

The DNA sequence of the 15,532-base pair (bp) mitochondrial DNA (mtDNA ) of the chiton Katharina tunicata has been determined. The 37 genes t ypical of metazoan mtDNA are present: 13 for protein subunits involved in oxidative phosphorylation, 2 for rRNAs and 22 for tRNAs. The gene arrangement resembles those of arthropods much more than that of anoth er mollusc, the bivalve Mytilus edulis. Most genes abut directly or ov erlap, and abbreviated stop codons are inferred for four genes. Four j unctions between adjacent pairs of protein genes lack intervening; tRN A genes; however, at each of these junctions there is a sequence immed iately adjacent to the start codon of the downstream gene that is capa ble of forming a stem-and-loop structure. Analysis of the tRNA gene se quences suggests that the D arm is unpaired in tRNA(ser(AGN)), which i s typical of metazoan mtDNAs, and also in tRNA(ser(UCN)), a condition found previously only in nematode mtDNAs. There are two additional seq uences in Katharina mtDNA that can be folded into structures resemblin g tRNAs; whether these are functional genes is unknown. All possible c odons except the stop codons TAA and TAG are used in the protein-encod ing genes, and Katharina mtDNA appears to use the same variation of th e mitochondrial genetic code that is used in Drosophila and Mytilus. T ranslation initiates at the codons ATG, ATA and GTG. A + T richness ap pears to have affected colon usage patterns and, perhaps, the amino ac id composition of the encoded proteins. A 142-bp non-coding region bet ween tRNA(glu) and CO3 contains a 72-bp tract of alternating A and T.