Paralogous Stellate and Su(Ste) repeats: evolution and ability to silence a reporter gene

The X-linked Stellate repeats, encoding a putative regulatory subunit of pr otein kinase CK2, are expressed in XO male testes. The Y-linked, testes-exp ressed paralogous Su(Ste) repeats are thought to be suppressors of Stellate transcription. The unique, testis-expressed euchromatic gene was suggested to be an ancestor of the both types of amplified paralogous repeats. A Su( Ste)-like orphon was localized on a Y chromosome, outside of the Su(Ste) cl uster. Several diagnostic molecular markers peculiar for the both types of diverged Stellate and Su(Ste) units were detected in the orphon sequence. T he orphon was suggested to be a close relative of the immediate ancestor of both types of paralogous repeats which initiated evolution on the Y chromo some. Selection pressure on the level of translation was shown as a driving force in the evolution of Su(Ste) repeats, which are considered as more an cient derivatives of the ancestor euchromatic gene than Stellate repeats. I n a vicinity of 12E Stellate cluster the undamaged, recently originated euc hromatic Stellate orphon was found at 12D, providing the poly(A) signal for the bendless gene. P-element mediated transformations reveal that the frag ments of cloned Stellate and Su(Ste) clusters are able to induce variegatio n of a reporter mini-white gene. The observed variegation phenomenon has pe culiar features: a significant increase of trans-activation of a reporter m ini-white gene in homozygous stat; absence of effects of several convention al modifiers of position effect variegation (PEV) and independence of a sev erity of variegation on a distance between insertion and centromere region.