Unusual organization of the 5S ribosomal spacer in Dirofilaria repens: absence of a canonical spliced leader 1 sequence.

Comparing the 5S ribosomal RNA spacer of Dirofilaria repens and D. immitis, we found that in D. repens PCR generates two amplification products. In both products, the putative spliced leader 1 sequences (SL1) show some nucleotide substitutions compared to the canonical SL1 sequence of nematodes. SL I is a short sequence transferred to the 5' end of most messenger RNAs through a trans-splicing reaction. All the other filariae studied thus far amplify a single fragment showing a canonical SL1 sequence. D. repens appears thus peculiar.