Cooperative evolution of two different TEs results in lineage-specific novel transcripts in the BLOC1S2 gene

The BLOC1S2 gene encodes the multifunctional protein BLOS2, a shared subunit of two lysosomal trafficking complexes: i) biogenesis of lysosome-related organelles complex-1 and i) BLOC-1-related complex. In our previous study, we identified an intriguing unreported transcript of the BLOC1S2 gene that has a novel exon derived from two transposable elements (TEs), MIR and AluSp. To investigate the evolutionary footprint and molecular mechanism of action of this transcript, we performed PCR and RT-PCR experiments and sequencing analyses using genomic DNA and RNA samples from humans and various non-human primates.

The results showed that the MIR element had integrated into the genome of our common ancestor, specifically in the BLOC1S2 gene region, before the radiation of all primate lineages and that the AluSp element had integrated into the genome of our common ancestor, fortunately in the middle of the MIR sequences, after the divergence of Old World monkeys and New World monkeys. The combined MIR and AluSp sequences provide a 3' splice site (AG) and 5' splice site (GT), respectively, and generate the Old World monkey-specific transcripts. Moreover, branch point sequences for the intron removal process are provided by the MIR and AluSp combination.

We show for the first time that sequential integration into the same location and sequence divergence events of two different TEs generated lineage-specific transcripts through sequence collaboration during primate evolution.