Efficient cleavage of HAC1 mRNA by Ire1* in the presence of cellular mRNA. (a) Schematic diagram of the Ire1*-cleaved (poly(A)-) and uncleaved (poly(A)+) RNA fractions separated after an in vitro nuclease reaction on yeast poly(A)+ RNA. (b) Northern blot of the RNA fractions indicated in (a) probed with a PCR fragment encompassing either the 5' exon of HAC1 (lanes 1-3) or the 3' exon (lanes 4-6). Lanes 1 and 4, yeast poly(A)+ RNA before Ire1* cleavage; lanes 2 and 5, bound uncleaved RNA fraction (b, poly(A)+); lanes 3 and 6, unbound cleaved RNA fraction (u, poly(A)-). Positions of uncleaved and cleaved HAC1 mRNA are indicated. Note that the 5' exon and 5' exon plus intron, and the 3' exon and 3' exon plus intron RNA species, respectively, co-migrate on these agarose gels.