GENE RPA43 IN SACCHAROMYCES-CEREVISIAE ENCODES AN ESSENTIAL SUBUNIT OF RNA-POLYMERASE-I

Yeast RNA polymerase I contains 14 distinct polypeptides, including A4 3, a component of about 43 kDa. The corresponding gene, RPA43, encodes a 326-amino acid polypeptide matching the peptidic sequence of two tr yptic fragments isolated from A43. Gene inactivation leads to a lethal phenotype that is rescued by a plasmid containing the S-35 ribosomal RNA gene fused to the GAL7 promoter, which allows the synthesis of S-3 5 rRNA by RNA polymerase II in the presence of galactose. A screening for mutants rescued by the presence of GAL7-(35)SrDNA identified a non sense rpa43 allele truncating the protein at amino acid position 217. [H-3]Uridine pulse labeling showed that this mutation abolishes S-35 r RNA synthesis without significant effects on the synthesis of 5 S RNA and tRNAs. These properties establish that A43 is an essential compone nt of RNA polymerase I. This highly hydrophilic phosphoprotein has a s trongly acidic carboxyl-terminal domain, and shows no homology to entr ies in current sequence data banks, including all the genetically iden tified components of the other two yeast RNA polymerases. RPA43 mapped next to RPA190, encoding the largest subunit of polymerase I, These g enes are divergently transcribed and may thus share upstream regulator y elements ensuring their co-regulation.