RAT1 encodes a nuclear 5'-3' exoribonuclease involved in rRNA processing (1, 5, 6).

RAT1 has been identified in several different screens, including a screen for temperature-sensitive mutations that cause defects in mRNA export (RAT1, (1)), mutations that activate Pol III transcription of a tRNA with a mutated promoter (TAP1,(6)), and mutations that are synthetically lethal with swi4 mutations (RSF11, (7)). The connection between these different genetic screens and RAT1 function has not yet been made.

Rat1p is 39% identical to the cytoplasmic Kem1p/Xrn1p exonuclease (1, 5, 6). Only dominant alleles of RAT1 were found in a screen for suppressors that rescued the viability of a kem1 ski2 double mutant strain; Ski2p is a putative RNA helicase (2). These dominant alleles of RAT1 mislocalized Rat1p to the cytoplasm. Overexpression of wildtype KEM1 does not suppress rat1-1 mutations; however, if Kem1p is targeted to the nucleus, it can substitute for Rat1p function, suggesting that Rat1p and Kem1p are functionally redundant proteins performing roles in separate cellular compartments (2). The rat1-1 allele exhibits temperature-sensitive defects in the formation of the 5' end of 5.8s rRNA, implicating Rat1p in processing of rRNA (1, 8). It was later shown that Rat1p and Kem1p are involved in processing pre-rRNA spacer fragments and snoRNAs as well (9).

Last updated: 2000-04-12