The yeast nuclear exosome contains multiple 3′→5′ exoribonucleases, raising the question of why so many activities are present in the complex. All components are required during the 3′ processing of the 5.8S rRNA, together with the putative RNA helicase Dob1p/Mtr4p. During this processing three distinct steps can be resolved, and hand‐over between different exonucleases appears to occur at least twice. 3′ processing of snoRNAs (small nucleolar RNAs) that are excised from polycistronic precursors or from mRNA introns is also a multi‐step process that involves the exosome, with final trimming specifically dependent on the Rrp6p component. The spliceosomal U4 snRNA (small nuclear RNA) is synthesized from a 3′ extended precursor that is cleaved by Rnt1p at sites 135 and 169 nt downstream of the mature 3′ end. This cleavage is followed by 3′→5′ processing of the pre‐snRNA involving the exosome complex and Dob1p. The exosome, together with Rnt1p, also participates in the 3′ processing of the U1 and U5 snRNAs. We conclude that the exosome is involved in the processing of many RNA substrates and that different components can have distinct functions.

The estrogen receptor α (ERα) is activated as a transcription factor by both estrogen and a large variety of other extracellular signals. The mechanisms of this ligand-independent activation, notably by cAMP signaling, are still largely unknown. We now close the gap in the signaling pathway between cAMP and ERα. Whereas the direct phosphorylation of ERα by the cAMP-activated protein kinase A (PKA) is dispensable, the phosphorylation of the coactivator-associated arginine methyltransferase 1 (CARM1) by PKA at a single serine is necessary and sufficient for direct binding to the unliganded hormone-binding domain (HBD) of ERα, and the interaction is necessary for cAMP activation of ERα. Sustained PKA activity promoting a constitutive interaction may contribute to tamoxifen resistance of breast tumors. Binding and activation involve a novel regulatory groove of the ERα HBD. As a result, depending on the activating signal, ERα recruits different coactivator complexes to regulate alternate sets of target genes.

The yeast exosome is a complex of 3′→5′ exonucleases involved in RNA processing and degradation. All 11 known components of the exosome are required during 3′ end processing of the 5.8S rRNA. Here we report that depletion of each of the individual components inhibits the early pre-rRNA cleavages at sites A₀, A₁, A₂ and A₃, reducing the levels of the 32S, 20S, 27SA₂ and 27SA₃ pre-rRNAs. The levels of the 27SB pre-rRNAs were also reduced. Consequently, both the 18S and 25S rRNAs were depleted. Since none of these processing steps involves 3′→5′ exonuclease activities, the requirement for the exosome is probably indirect. Correct assembly of trans-acting factors with the pre-ribosomes may be monitored by a quality control system that inhibits pre-rRNA processing. The exosome itself degrades aberrant pre-rRNAs that arise from such inhibition. Exosome mutants stabilize truncated versions of the 23S, 21S and A₂-C₂ RNAs, none of which are observed in wild-type cells. The putative helicase Dob1p, which functions as a cofactor for the exosome in pre-rRNA processing, also functions in these pre-rRNA degradation activities.

RNA-binding proteins of the L7Ae family are at the heart of many essential ribonucleoproteins (RNPs), including box C/D and H/ACA small nucleolar RNPs, U4 small nuclear RNP, telomerase, and messenger RNPs coding for selenoproteins. In this study, we show that Nufip and its yeast homologue Rsa1 are key components of the machinery that assembles these RNPs. We observed that Rsa1 and Nufip bind several L7Ae proteins and tether them to other core proteins in the immature particles. Surprisingly, Rsa1 and Nufip also link assembling RNPs with the AAA + adenosine triphosphatases hRvb1 and hRvb2 and with the Hsp90 chaperone through two conserved adaptors, Tah1/hSpagh and Pih1. Inhibition of Hsp90 in human cells prevents the accumulation of U3, U4, and telomerase RNAs and decreases the levels of newly synthesized hNop58, hNHP2, 15.5K, and SBP2. Thus, Hsp90 may control the folding of these proteins during the formation of new RNPs. This suggests that Hsp90 functions as a master regulator of cell proliferation by allowing simultaneous control of cell signaling and cell growth.

Very few of the enzymes required for eukaryotic precursor ribosomal RNA (pre-rRNA) processing have been identified. Ribonuclease (RNase) MRP was characterized as a nuclease that cleaves mitochondrial replication primers, but it is predominantly nucleolar. Previous genetic evidence revealed that this ribonucleoprotein is required, directly or indirectly, for cleavage of the yeast pre-rRNA in vivo at site A ₃ . Here, an in vitro processing system that accurately reproduces this cleavage is described. Biochemical purification and the use of extracts depleted of the MRP RNA demonstrate that endonucleolytic cleavage of the pre-rRNA is directly mediated by RNase MRP. This establishes a role for RNase MRP in the nucleolus.