Distribution and consensus of branch point signals in eukaryotic genes: a computerized statistical analysis.

An intermediate stage in the process of eukaryotic RNA splicing is the formation of a lariat structure. It is anchored at an adenosine residue in intron between 10 and 50 nucleotides upstream of the 3' splice site. A short conserved sequence (the branch point sequence) functions as the recognition signal for the site of lariat formation. It has been generally assumed that the branch point is recognized mainly by the presence of its unique sequence where the lariat is formed. However, the known branch point consensus sequence is found to be distributed nearly randomly throughout the gene sequence with only a slightly higher frequency in the expected lariat region. Further, the known consensus sequence is found to be clearly inadequate to specify branch points. These observations have implications for understanding the mechanism of branch point recognition in the process of splicing, and the possible evolution of the branch point signal.