ribosome production. Delays in posttranscriptional steps of ribosome maturation after TOR inactivation, as indicated by the relative accumulation of the 35S rRNA precursor (Figures 4D and 5D) and the re-localization of certain ribosome biogenesis factors (53,54), could explain these obvious diﬀerences. In conclusion, our work investigating the eﬀects of Rrn3p level alterations has revealed that the endogenous Rrn3p level is a determinant for the formation of initiation competent Pol I–Rrn3p complexes, inﬂuencing the (re-)initiation rate at rRNA genes. Rrn3p levels can thus directly inﬂuence Pol I transcription, which may be im- portant for long-term adjustment of ribosome synthesis. TOR inactivation also reduces Rrn3p levels but we provide clear evidence that this reduction cannot explain the dramatic decrease in mature rRNA production (Figure 5D). Accordingly, artiﬁcial down-regulation of Rrn3p slows down but does not inhibit cell growth, whereas TOR inactivation results in immediate growth arrest. This all supports the idea, that under certain physiological situations down-regulation of rRNA pro- duction can be uncoupled from eﬃcient Pol I initiation complex formation. Therefore, a main target of TOR in- activation in yeast to quickly adjust ribosome biosynthesis to changes in environmental conditions is likely down- stream of transcription initiation (see also Reiter et al., submitted). SUPPLEMENTARY DATA Supplementary Data are available at NAR Online. ACKNOWLEDGEMENTS The authors thank Dr E. Herrero (Lleida) for plasmids containing TetO-promoters, Dr D. Wolf (Stuttgart) for providing yeast strains YWO365 and cim3-1, Drs Ch. Carles and M. Riva (Gif-sur-Yvette) for antibodies, Dr E. Hurt for plasmid pNOP-GFP and S. Ferreira-Cerca for technical advice with the RNA analysis and for discussions. 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