Résumé
U6 is the most highly conserved spliceosomafl snRNA. Previous mutational studies have shown that the majority of essential residues in U6 are located in a region of 35 nucleotides encompassing a conserved hexanucleotide and stem I and stem II of the U4-interaction domain. Although the yeast and human U6 RNAs are 80% identical in this region, the human US gene cannot functionally replace the yeast gene in vivo. The human gene is not transcribed when placed in the context of yeast flanking sequences. Transcription of the human gene, but not its function, can be stimulated by the introduction of an A block promoter element in the 06 coding region. Using a set of human-yeast chimeras, we show that the 5' domain and the 3' terminal region of the human US gene can each functionally replace the corresponding yeast domains. However, a combination of both domains in a single molecule is lethal. The basis of the inability of the human US snRNA to function in yeast cells is discussed.