TRANS-SPLICEOSOMAL U6 RNAS OF CRITHIDIA-FASCICULATA AND LEPTOMONAS-SEYMOURI - DEVIATION FROM THE CONSERVED ACAGAG SEQUENCE AND POTENTIAL BASE FAIRING WITH SPLICED LEADER RNA

U6 RNA genes from the trypanosomatids Crithidia fasciculata and Leptom onas seymouri have been isolated and sequenced. As in Trypanosoma bruc ei, the U6 RNA genes in both C. fasciculata and L. seymouri are arrang ed in close linkage with upstream tRNA genes. The U6 RNA sequences fro m C. fasciculata and L. seymouri deviate in five and three positions, respectively, from the published T. brucei sequence. Interestingly, bo th C. fasciculata U6 RNA genes carry a C-->T change at the second posi tion of the ACAGAG hexanucleotide sequence, which is important for spl icing function and has been considered phylogenetically invariable. A compensatory base change of the C. fasciculata spliced leader RNA at t he highly conserved 5' splice site position +5, G-->A, suggests that a n interaction between the 5' splice site region and U6 RNA recently pr oposed for the yeast cis-splicing system may also occur in trans splic ing.