SUMMARY PARAGRAPH for RPA135
Nuclear transcription in S. cerevisiae is performed by three multisubunit nuclear RNA polymerases (RNAPs) that are conserved in all eukaryotes (6, 7 and references therein). The roles of these three RNA polymerases are generally conserved across eukaryotes, particularly with respect to production of rRNAs, mRNAs, and tRNAs, though production of other small RNAs is somewhat variable between RNAP II and RNAP III in different species (8). In S. cerevisiae, RNA polymerase I transcribes rDNA to produce the 35S primary rRNA transcript that is processed to produce three of the four mature ribosomal rRNAs: 25S, 18S, and 5.8S. RNA polymerase II produces all nuclear mRNAs, all of the snoRNAs except snR52 (9), four of the five snRNAs (U1, U2, U4, and U5; 10), the RNase MRP RNA encoded by NME1 (8), and the telomerase RNA encoded by TLC1 (11). RNA polymerase III produces the 5S rRNA, all nuclear tRNAs, the U6 snRNA (12), the snR52 snoRNA (9), the RNase P RNA encoded by RPR1 (8), and the 7SL RNA component of the signal recognition particle encoded by SCR1 (8).
Coordinate regulation of these three RNA polymerases is essential, since in rapidly growing yeast cells, much of the transcriptional output of the cell is devoted to the production of ribosomes. About 60% of total cellular transcription is devoted to transcription by RNAP I of the rRNA genes, which comprise about 10% of the entire genome. While mRNAs generally only comprise 5% of total cellular RNA and the 137 ribosomal protein (RP) genes represent only 2% of the genome, it is estimated that 50% of RNAP II transcription occurs on RP genes. RNAP II is also responsible for production of the majority of the snoRNAs, which are collectively involved in maturation of the ribosome. RNAP III plays a similarly important role in production of ribosomes and the process of translation, producing both the 5S rRNA and all nuclear tRNAs, which constitute about 15% of total cellular RNA (reviewed in 13). The TOR pathway is a major factor in this coordinate regulation as it regulates the activity of all three nuclear RNAPs in response to nutrient availability and growth conditions (reviewed in 14, 15, 16, and 17).
In addition to producing the majority of cellular RNA, RNAP I and RNAP III may also play roles in nuclear architecture and genome organization. RNAP I activity may be involved in organizing the rDNA repeats into the nucleolus (reviewed in 18). Active tRNA genes transcribed by RNAP III appear to act as chromatin boundary elements that affect both transcription and DNA replication. Additionally, recombination between dispersed tRNA genes may be a source of genetic instability and evolutionary change (reviewed in 19).
Five genes (RPB5, RPO26, RPB8, RPB10, and RPC10) encode subunits common to all three of the nuclear RNA polymerases. Two genes (RPC40 and RPC19) encode subunits present in both RNAP I and RNAP III; RPB3 and RPB11 encode the corresponding RNAP II subunits. Five more subunits are encoded by a separate gene for each polymerase, but are considered functional equivalents of each other. Thus there are twelve subunits that are conserved in all three of the nuclear RNA polymerases, eleven of which correspond to subunits of Archaeal RNAPs, and five of which also correspond to the subunits of E. coli RNAP. In each, ten of these comprise the enzyme cores, while Rpb4/7 (RNAP II), Rpa14/43 (RNAP I), and Rpc17/25 (RNAP III) form heterodimers which associate with this core and have roles in initiation (20). RNAPs I and III also have two subunits which are homologous to the subunits of the TFIIF general initiation factor for RNAP II, and RNAP III has three additional unique subunits (reviewed in 21, 7, and 22). For tables showing the correspondence between the subunits of the three nuclear RNA polymerases in S. cerevisiae see Cramer et al. 2008 (22) and Werner et al. 2009 (21); to see the correspondence with those of Archaea and bacteria see Cramer 2002 (7).
About RNA polymerase I
In S. cerevisiae, the RNA polymerase I enzyme is composed of fourteen subunits. RPB5, RPO26, RPB8, RPC10, RPB10, RPC40, and RPC19 encode subunits shared with one or both of the other two nuclear RNA polymerases. RPA49 and RPA34 encode subunits with counterparts in RNA polymerase III and RPA190, RPA135, RPA43, RPA14, and RPA12 encode subunits with counterparts in both RNA polymerases II and III (23, 24, and reviewed in 20).
Most of the genes encoding subunits of RNA polymerase I are essential (25 and references therein) and elegant genetic experiments have shown that production of the large rRNA transcript is the only essential role of these genes (reviewed in 26). However, null mutations in any of four of the genes (RPA49, RPA34, RPA14, and RPA12) encoding subunits present only in RNAP I produce viable strains. While a triple mutant strain lacking functional RPA49, RPA34, and RPA12 is viable, inactivating any one of these genes in combination with RPA14 is lethal. Thus these four subunits are dispensible individually but collectively become essential (27). Rpa49p and Rpa34p, as expected from their similarity to TFIIF, contribute to the elongation properties of RNAP I (20). Rpa12p contains a C-terminal domain with similarity to the RNAP II elongation factor TFIIS (encoded by DST1) which appears to activate the transcript cleavage activity intrinsic to the RNAP I catalytic core (20). Mutations in core subunits such as RPA190, RPA135, RPC40, and RPC19 often affect the basic functions of core enzyme assembly and catalytic properties of initiation, elongation, or termination, as well as the association of the core enzyme with the other complexes required for RNAP I function in vivo (25 and references therein).
RNAP I transcription requires a number of factors in addition to the polymerase itself: TATA-binding protein (TBP, encoded by SPT15), the initiation factor Rrn3 (homologous to mammalian TIF-IA), the core factor CF (composed of Rrn6p, Rrn7p, and Rrn11p), and the upstream activating factor UAF (composed of Rrn5p, Rrn9p, Rrn10p, Uaf30p, and histones H3 and H4). While some of these factors have mammalian homologs, others are more diverged, as might be expected from the fact that there is little conservation of rDNA promoter sequences across taxonomic groupings although some structural elements are conserved (reviewed in 26 and 14). UAF binds to the promoter and recruits CF and a complex of Rrn3p associated with RNAP I. Rrn3p plays a key role in the regulation of RNAP I activity, as RNAP I is only able to initiate transcription when it is associated with Rrn3p, but any of the RNAP I transcription factors may serve as a target for regulation. In addition, the TFIIH factor, originally characterized as a RNAP II transcription factor, is also required for productive transcriptional elongation by RNAP I and for coupling of DNA repair to rDNA transcription. Numerous regulatory pathways are involved in the complex regulation of RNAP I in response to growth signals, including both the TOR and MAP kinase signaling pathways and chromatin remodeling activities (reviewed in 15, 14 and 17). Thus control of RNAP I activity is central to control of ribosome production and growth control in S. cerevisiae.
Last updated: 2010-04-29