Double strand RNA mediated silencing as a mechanism evolved to suppress Stellate transcription and male sterility. A.A. Aravin , A.V. Tulin , V.A. Gvozdev. Institute of Molecular Genetics, 123182, Kurchatov sq, Moscow, Russia.

   The testis-specific expression of X-linked Stellate repeats encoding putative regulatory subunit of protein kinase CK2 are suppressed by closely homologous Y-linked Su(Ste) repeats. Stellate hyperexpression due to the absence of the Y chromosome or a deletion of a bulk of Su(Ste) repeats cause male sterility.
    Sense transcription of Su(Ste) repeats was shown earlier (Kalmykova et al 1998, Genetics 148: 243-249). Using strand-specific RT-PCR and Northern-analysis we detected antisense transcription of Su(Ste) repeats, which is greatly decreased because of a deletion of a bulk of Su(Ste) repeats. No antisense Stellate transcription was revealed. The start of antisense transcription is suggested to be located in hoppel transposon inserted in the each Su(Ste) repeat. Mutation in the sting gene, whose C. elegans rde-1 homolog participate in dsRNA-mediated interference, causes hyperexpression of Stellate genes in the presence of intact Su(Ste) locus, but exerts no influence on a quantity of Su(Ste) antisense transcripts. RT-PCR analysis confirmed earlier observation that Stellate derepression is followed by a dramatic increase of a quantity of spliced transcripts whereas amount of unspliced transcripts remains unchanged (Livak 1990, Genetics 124:303-316). Thus, one could imagine mechanism of suppression based on splicing regulation. However, expression of fused Ste-lacZ construction containing the start point of Stellate transcription and providing synthesis of intronless portion (40 bp) of Stellate RNA was shown to be suppressed by Su(Ste) repeats. Therefore it seems that Ste/Su(Ste) regulation doesn’t change quantity of non-spliced transcripts and proceeds downstream of splicing as was shown for dsRNA-mediated interference in C. elegans. We propose that the mechanism of Ste/Su(Ste) regulation based on dsRNA-mediated silencing was evolved as a result of hoppel transposon insertion in Su(Ste) repeats.