MOLECULAR-CLONING AND COMPLETE NUCLEOTIDE-SEQUENCE OF THE REPEATED UNIT AND FLANKING GENE OF THE SCALLOP PECTEN-MAXIMUS MITOCHONDRIAL-DNA -PUTATIVE REPLICATION ORIGIN FEATURES

In the bivalve mollusc Pecten maximus, the size of the mitochondrial D NA molecules ranges from 20 to 25.8 kbp. This variability is mainly co rrelated with the occurrence of a variable domain composed with two to five 1.6-kbp repeated units tandemly arrayed in the genome. DNA fragm ents spanning the 1,586-base-pair-long repeated element and the neares t flanking gene have been cloned and sequenced. This sequence was anal yzed regarding its base composition and potential secondary structures . The repeated unit domain was positioned and oriented with regard to the known flanking gene. It ends 2 base pairs upstream relative to the beginning of the tRNA(gly) gene. The peculiar properties of the repea ted unit were compared with those of the 1,442-bp repeated element fou nd in the mitochondrial genome of the deep sea scallop Placopecten mag ellanicus. This comparison provided evidence for the absence of nucleo tide conservation, except for a small sequence engaged in a secondary structure, but argued for a strong pressure maintaining domains with s pecific nucleotide content. A possible role for the conserved sequence is discussed.