Transcription factor that regulates ribosomal protein (RP) and ribosome biogenesis (Ribi) gene expression in response to nutrients and stress. Promotes RP gene expression under optimal growth conditions. Leaves the nucleus upon environmental challenges, resulting in a down-regulation of RP gene transcription. The effect of the environmental cues on SFP1 localization is mediated through the TOR pathway. Also regulates the expression of genes involved in the G2/M transition during the mitotic cell cycle and the DNA-damage response. Required for carbon-source modulation of cell size. Interacts with the target of rapamycin complex 1 (TORC1) in a rapamycin-dependent manner. Interacts with MRS6. Nuclear under optimal growth conditions. Leaves the nucleus in response to stress or changes in nutrient availability. The [ISP+] aggregates appear to be nuclear. The prion domain (PrD) is a Gln/Asn (Q/N)-rich domain, which is unstructured in its native, soluble form, and which forms a parallel in-register beta-sheet in its amyloid form. Phosphorylated by TORC1 kinase at multiple sites. Phosphorylation regulates nuclear localization and RP promoter binding. [ISP+] is the prion form of SFP1. [ISP+] is the result of a conformational change of the cellular SFP1 protein that becomes self-propagating and infectious. This conformational change generates a form of SFP1 that assembles into amyloid fibrils. [ISP+]-aggregates accumulate in the nucleus, and results in significantly larger cell size and increased drug resistance (PubMed:20498075). [ISP+] can be cured by GdnHCl (PubMed:11805042). It is speculated that prion properties of transcription factors may generate an optimized phenotypic heterogeneity that buffers yeast populations against diverse environmental insults (PubMed:20498075). Present with 259 molecules/cell in log phase SD medium. It is uncertain whether Met-1 or Met-6 is the initiator.