J. Delacruz et al., DOB1P (MTR4P) IS A PUTATIVE ATP-DEPENDENT RNA HELICASE REQUIRED FOR THE 3' END FORMATION OF 5.8S RIBOSOMAL-RNA IN SACCHAROMYCES-CEREVISIAE, EMBO journal, 17(4), 1998, pp. 1128-1140
The temperature-sensitive mutation, dob1-1, was identified in a screen
for dependence on overexpression of the yeast translation initiation
factor eIF4B (Tif3p). Dob1p is an essential putative ATP-dependent RNA
helicase. Polysome analyses revealed an under accumulation of 60S rib
osomal subunits in the dob1-1 mutant. Pulse-chase labelling of pre-rRN
A showed that this was due to a defect in the synthesis of the 5.8S an
d 25S rRNAs. Northern and primer extension analyses in the dob1-1 muta
nt, or in a strain genetically depleted of Dob1p, revealed a specific
inhibition of the 3' processing of the 5.8S rRNA from its 7S precursor
. This processing recently has been attributed to the activity of the
exosome, a complex of 3'-->5' exonucleases that includes Rrp4p. In viv
o depletion of Dob1p also inhibits degradation of the 5' external tran
scribed spacer region of the pre-rRNA. A similar phenotype was observe
d in rrp4 mutant strains and, moreover, the dob1-1 and rrp4-1 mutation
s show a strong synergistic growth inhibition. We propose that Dob1p f
unctions as a cofactor for the exosome complex that unwinds secondary
structures in the pre-rRNA that otherwise block the progression of the
3'-->5' exonucleases.