SEX-SPECIFIC AND TISSUE-SPECIFIC BKM(GATA)-BINDING PROTEIN IN THE GERM-CELLS OF HETEROGAMETIC SEX

The ZZ male/ZW female system of sex determination (female heterogamety ) is found in snakes and birds whereas XY male/XX female system of sex determination (male heterogamety) operates in mammals including human s. The W and Y chromosomes are largely heterochromatic and undergo cyc les of condensation and decondensation in the germ cells of ovary and testis, respectively, whereas they remain highly condensed and transcr iptionally inactive in all somatic cells. Both chromosomes have enrich ed stretches of GATA repeats along their entire length (which is ident ified as banded krait minor satellite DNA and called Bkm) that are hig hly conserved through widely separated orders of eukaryotes. Here we r eport the existence of a factor, which specifically binds to Bkm, in t he germ cells of the heterogametic sex (ovary in female heterogamety a nd testis in male heterogamety) where decondensation (activation) of t he W and Y chromosomes, respectively, occurs; it has been purified as a polypeptide of 57.5 kDa from the rat snake ovary and designated as B km-binding protein (BBP) by virtue of its binding to GATA repeats of B km. Such a sex- and tissue specific BBP is also present in the ovary o f other species of snakes and in the testis of mouse and human where t he Y chromosome is highly decondensed. We suggest that GATA repeats of Bkm brings about a coordinated decondensation of the W and Y sex chro mosomes in the germ cells of the heterogametic sex in response to BBP which may serve as a ''switch'' for the activation of the genes presen t on the W and Y chromosomes.