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Unusual branch point selection in processing of human growth hormone pre-mRNA

Intron A of the human growth hormone gene does not contain an A residue within 56 nucleotides preceding the 3' splice site. The analysis of the excised intron lariat revealed a C residue 28 nucleotides upstream from the 3' splice site as the major branch acceptor nucleotide. Two additional minor branched nucleotides were identified as U residues at positions -22 and -36. An adenosine substitution at position -22 results in lariat formation solely to this nucleotide. Therefore, C and U residues can function efficiently as natural branch acceptors, but an A residue is preferred if available in the proper region. In addition, the data strongly reinforce the importance of the distance constraint for lariat formation. To explain selection of the branch acceptor nucleotide, potential base-pairing interactions of branch point sequences with the U2 RNA are discussed.