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Abstract
The immunoglobulin M heavy-chain locus contains two poly(A) sites which are alternatively expressed during B-cell differentiation. Despite its promoter proximal location, the secretory poly(A) site is not expressed in undifferentiated cells. Crucial to the activation of the secretory poly(A) site during B-cell differentiation are changes in the binding of cleavage stimulatory factor 64K to GU-rich elements downstream of the poly(A) site. What regulates this change is not understood. The secretory poly(A) site contains two downstream GU-rich regions separated by a 29-nucleotide sequence. Both GU-rich regions are necessary for binding of the specific cleavage-polyadenylation complex. We demonstrate here that U1A binds two (AUGCN1-3C) motifs within the 29-nucleotide sequence and inhibits the binding of cleavage stimulatory factor 64K and cleavage at the secretory poly(A) site.
We thank Yoshio Takagaki for the GSTCstF64KRBD plasmid, Chris Milcarek and Ravinder Singh for advice, and Steve Jung for technical assistance.
The work was supported by AHA 0430004N Scientist Development Grant to C. Phillips and NIH GM57286 to S. Gunderson.