The 5' splice site: phylogenetic evolution and variable geometry of association with U1RNA

The 5' splice site sequences of 3294 introns from various organisms (1-672) were analyzed in order to determine the rules governing evolution of this sequence, which may shed light on the mechanism of cleavage at the exon-intron junction. The data indicate that, currently, in all organisms, a common sequence 1GUAAG6U and its derivatives are used as well as an additional sequence and its derivatives, which differ in metazoa (G/1GUgAG6U), lower eucaryotes (1GUAxG6U) and higher plants (AG/1GU3A). They all partly resemble the prototype sequence AG/1GUAAG6U whose 8 contigous nucleotides are complementary to the nucleotides 4-11 of U1RNA, which are perfectly conserved in the course of phylogenetic evolution. Detailed examination of the data shows that U1RNA can recognize different parts of 5' splice sites. As a rule, either prototype nucleotides at position -2 and -1 or at positions 4, 5 or 6 or at positions 3-4 are dispensable provided that the stability of the U1RNA-5' splice site hybrid is conserved. On the basis of frequency of sequences, the optimal size of the hybridizable region is 5-7 nucleotides. Thus, the cleavage at the exon-intron junction seems to imply, first, that the 5' splice site is recognized by U1RNA according to a "variable geometry" program; second, that the precise cleavage site is determined by the conserved sequence of U1RNA since it occurs exactly opposite to the junction between nucleotides C9 and C10 of U1RNA. The variable geometry of the U1RNA-5' splice site association provides flexibility to the system and allows diversification in the course of phylogenetic evolution.