The Xenopus YB3 protein binds the B box element of the class III promoter

We have isolated a Xenopus cDNA encoding the YB3 protein which binds specifically to the B box promoter element of class III genes. Northern analysis shows YB3 is expressed in a variety of adult tissues. Fractionation of oocyte S150 extracts demonstrates YB3 is present in phosphocellulose fraction IIIC, as well as in the fraction isolated by B box DNA affinity chromatography. Silver staining indicates that YB3, or a protein of the same mobility in SDS gels, is the most abundant component in either fraction.