PHYLOGENETIC CONSERVATION OF MODIFIED NUCLEOTIDES IN THE TERMINAL LOOP-1 OF THE SPLICEOSOMAL U5 SNRNA

In order to study the phylogenetic conservation of modified nucleotide s in the spliceosomal U5 snRNA, we determined the nucleotide sequences of the U5 snRNAs from the slime mold Physarum polycephalum (EMBL data bank accession numbers: X74440 and X74441) and we identified the pseu douridine and 2'-O-methylated residues. From a comparison of all the U 5 snRNAs studied at the level of nucleotide modifications, we conclude d that the modified nucleotides in U5 snRNA can be divided into three classes according to their degree of conservation: i) the modified nuc leotides of the 5' terminal cap structure that display some variations from one species to the other; ii) the modified nucleotides located i n the helical part of the stem/loop structure I that vary greatly in n umber, position and identity from one species to the other; and iii) t he modified nucleotides of the terminal loop 1, that are almost identi cal in all the species studied. Taking into account the recent discove ry of a crucial role played by this terminal loop of U5 snRNA in 5' an d 3' splice site definition, we postulate that the numerous modified n ucleotides it contains, five out of a total of 11, play an important r ole in spliceosome assembly and function. Their possible role is discu ssed.