Ribosomal RNA (rRNA) transcription is a key step in the synthesis of ribosomes and occurs through the control of RNA polymerase I (Pol I; Grummt, 2003
; Moss, 2004
; Russell and Zomerdijk, 2005
). Over the last several decades, studies in unicellular systems, particularly Escherichia coli
and Sacharomyces cerevisiae
, have examined the mechanisms by which rRNA synthesis is regulated (Nomura, 1999
; Paul et al., 2004
). More recent studies in mammalian cell culture have also identified mechanisms via which Pol I is controlled. However, few studies have addressed how rRNA synthesis is regulated in vivo during the growth of a multicellular animal. Given that ribosome biogenesis links extracellular signals to the control of cell growth, identifying the mechanisms that operate in vivo should provide key insights into the control of cell and tissue growth.
Nutrient availability is a key determinant of cell and organismal growth. In eukaryotes, the target of rapamycin (TOR) kinase pathway is a major growth–regulatory pathway activated in response to nutrient availability (Wullschleger et al., 2006
). Although biochemical and genetic analyses have defined the signaling inputs to TOR, the outputs via which TOR drives growth are not fully understood. An extensive literature suggests that TOR controls growth by stimulating mRNA translation, particularly through the effectors ribosomal protein (RP), S6 kinase (S6K), and translation initiation factor 4E–binding protein
. But these targets are unlikely to explain all the effects of TOR in vivo. For example, Drosophila melanogaster TOR
mutants are lethal and have marked growth defects, whereas S6K
and 4E-binding protein
mutants are viable and have mild growth phenotypes (Montagne et al., 1999
; Oldham et al., 2000
; Zhang et al., 2000
; Miron et al., 2001
). Hence, other downstream targets and metabolic processes must additionally contribute to the effects of TOR in vivo.
Studies in yeast and mammalian cell culture indicate that regulation of rRNA synthesis is a conserved TOR function (Zaragoza et al., 1998
; Powers and Walter, 1999
; Hannan et al., 2003
; Tsang et al., 2003
; Claypool et al., 2004
; James and Zomerdijk, 2004
; Mayer et al., 2004
; Li et al., 2006
). A few studies have described mechanisms by which TOR can affect Pol I activity; however, these have yielded conflicting results. Studies in yeast and mammalian cell culture reported that TOR regulated the ability of a conserved transcription factor, transcription initiation factor IA (TIF-IA; or Rrn3p, the yeast homologue of TIFI-IA), to recruit Pol I to rDNA (Claypool et al., 2004
; Mayer et al., 2004
). But another paper on mammalian cells found that TIF-IA is dispensable for TOR-dependent regulation of rRNA synthesis and suggested that a different Pol I factor, upstream binding factor (UBF), was a target of TOR signaling (Hannan et al., 2003
). Finally, a recent paper showed that TOR associates with the rDNA in yeast, suggesting that regulation of Pol I by TOR is direct (Li et al., 2006
). TOR may therefore control rRNA synthesis through multiple mechanisms. Whether or not TOR regulates rRNA synthesis in vivo in animals and what mechanisms may operate in this context have not been examined.
Here, we examine the role of the D. melanogaster homologue of the conserved Pol I factor TIF-IA in the control of ribosome synthesis and growth. We show that TIF-IA is required for rRNA synthesis and cell and organismal growth and that TIF-IA functions downstream of the TOR pathway in vivo. We also provide evidence that stimulation of rRNA synthesis by TIF-IA can control the levels of other ribosome components.