mRNP4, a major mRNA-binding protein from Xenopus oocytes is identical to transcription factor FRG Y2

Previous work has revealed a striking similarity between the sequence of mRNP4, a major mRNA-binding protein of Xenopus laevis oocytes and FRG Y2, a transcription factor specific for Y-box-containing promoters. However, the apparent molecular mass of mRNP4 exceeds by 50-60% the cDNA-deduced molecular mass of FRG Y2. To resolve this discrepancy we have measured the real molecular mass of mRNP4 by sedimentation equilibrium. The molecular mass of mRNP4 closely agrees with that of FRG Y2. We conclude that mRNP4 is identical to FRG Y2 and has a dual function in oocytes. As a DNA-binding protein, mRNP4 stimulates transcription from a specific set of promoters. As a mRNA-binding protein, mRNP4 favors accumulation of the transcripts it contributes to produce.