The cyc1-512 mutation results in a 90% reduction in the
levels of the CYC1 mRNA and protein because of the lack of a
proper 3' end forming signal. Analysis of cyc1-512 revertants
revealed that they either contained new 3' end forming signals or were
due extragenic suppressors that enhanced the levels of the cyc1-512 mRNAs because of diminished degradation. The suppressors
constituted recessive mutations that could be assigned to at least two
unessential genes, CBC1 , which encodes the CBP80 subunit of the
nuclear cap binding complex, and UPF1 , which encode a major
component of the mRNA survellance complex. The requirements of Cbc1p and
Upf1p for mRNA degradation was investigated with cbc1 and
upf1 deletion mutants, and by examining the degradation of
cyc1-512 , ACT1 and CYH2 and total mRNAs as well as
CYH2 pre-mRNA. The Cbc1p-dependent degradation of total mRNA was
shown to occur in the cytosol by FISH (fluorescent in situ
hybridization) analysis of RAT7 CBC1 , RAT7
cbc1 , rat7-1 CBC1 and rat7-1 cbc1
strains, in which there is a rapid cessation of mRNA in rat7-1
strains at the restricted temperature. The relative degradation of
cyc1-512 mRNA in cbc1 and upf1 single and double
mutant strains suggest that Cbc1p and Upf1p may be components of the
same functional complex required for mRNA degradation, but the lack of
either of these components diminishes degradation of different classes
of mRNAs, such that both are required for cyc1-512 mRNA
degradation, whereas Upf1p is required for efficient degradation of
nonsense-mediated mRNA decay and Cbc1p is required for efficient
degradation normal mRNAs.
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