Sequence characterization of a unique intergenic spacer in Gadiformes mitochondrial DNA

The nucleotide sequences of intergenic spacers located between the tRNA(Thr ) and tRNA(Pro) genes in mitochondrial DNA of cod fishes (order Godiformes) were determined. Spacers from eight species representing two families of c od fishes were analyzed and found to vary in size from 25 to 99 bp. Each sp acer sequence contains one or two copies of a conserved 17-bp motif. Four t o five central nucleotides of this motif constitute a substitutional hot sp ot as observed from interspecific and intraspecific comparisons. The substi tution rate of the spacer is approximately twice that of the variable part I of the mitochondrial DNA control region, making this sequence region inte resting as a molecular marker in population studies or stock assessments of cod fishes. We propose that the spacer originated in a duplication event a nd evolved into a functional domain, perhaps by binding regulatory proteins .