Two genes become one: The genes encoding heterochromatin protein SU(VAR)3-9 and translation initiation factor subunit eIF-2 gamma are joined to a dicistronic unit in holometabolic insects

The Drosophila suppressor of position-effect variegation Su(var)3-9 encodes a heterochromatin-associated protein that is evolutionarily conserved. In contrast to its yeast and mammalian orthologs, the Drosophila Su(var)3-9 ge ne is fused with the locus encoding ale gamma subunit of translation initia tion factor eIF2. Synthesis of the two unrelated proteins is resolved by al ternative splicing. A similar dicistronic Su(var)3-9/elF-2y transcription u nit was found in Clytus arietis, Leptinotarsa decemlineata, and Scoliopteri x libatrix, representing two different orders of holometabolic insects (Col eoptera and Lepidoptera). In all these species the N terminus of the eIF-2y , which is encoded by the first two exons, is fused to SU(VAR)3-9. In contr ast to Drosophila melanogaster, RT-PCR analysis in the two coleopteran and the lepidopteran species demonstrated the usage of a nonconserved splice do nor site located within the 3' end of the SU(VAR)3-9 ORF, resulting in remo val of the Su(var)3-9-specific stop codon from the mRNA and complete in-fra me fusion of the SU(VAR) 3-9 and eIF-2y ORFs. In the centipede Lithobius fo rficatus eIF-2y and Su(var)3-9 are unconnected. Conservation of the dicistr onic Su(var)3-9/eIF-2y transcription unit in the studied insects indicates its origin before radiation of holometabolic insects and represents a usefu l tool for molecular phylogenetic analysis in arthropods.