Deletion analysis of a unique 3' splice site indicates that alternating guanine and thymine residues represent an efficient splicing signal.

The 3' splice site of the second intron (I2) of the human apolipoprotein-AII gene, (GT)16GGGCAG, is unique in that, although fully functional, a stretch of alternating guanine and thymine residues replaces the polypyrimidine tract usually associated with 3' splice junctions. The transient expression of successive 5' deletion mutants has defined the minimum number of nucleotides at the 3' end of apo-AII I2 that are required to direct efficient splicing. Processing in two cell-types, representing apo-AII producing and non-producing tissue was identical; in both, only by removing all the GT repeats did the 3' splice site of apo-AII I2 become completely non-functional. Similar deletion analyses of "classic" 3' splice sites, which conform to the consensus sequence (Y)nNYAG, have indicated that a minimum of 14 nucleotides of the polypyrimidine tract are required for detectable levels of processing to take place. Here we report that the six nucleotides (GT)2GG, which directly replace this tract in a deletion mutant of the 3' splice site of apo-AII I2 are sufficient to direct the splicing process efficiently and correctly.