Crystallographic data on the structure of Qnr-like PRPs such as MfpA, EfsQnr and AhQnr, revealed a right handed b helix structure that mimics a double-strain helix DNA. This could permit interaction with other DNA-linked proteins with a possible regulating function of the activity of these proteins. Mimicking DNA structure could have consequences on cell cycle and replication. Thus, this could explain the impact on fitness. Rapamycin is an immunosuppressant and an anticancer molecule that acts through inhibition of the TOR signaling pathway. In the yeast Saccharomyces cerevisiae, TOR1 and TOR2 encode serine/threonine kinases that form the core of the rapamycin sensitive and the rapamycin insensitive complex, respectively. TORC1 positively regulates anabolic processes by promoting mRNA translation and the transcription of ribosome biogenesis genes. Upon nutrient starvation or rapamycin treatment, the TORC1 complex becomes inactivated, with the consequence of a severe reduction of anabolic processes, as well as the induction of catabolic processes and stress responsive factors. These drastic changes are driven by important alterations of gene transcription AbMole Scopoletin mediated, at least in part, by the translocation of transcription factors to the nucleus. Upon TORC1 inhibition, the ribosomal gene repressor, Crf1, translocates into the nucleus to inhibit ribosomal gene transcription. Furthermore, the TORC1 regulator Tap42 activates PP2A and Sit4 phosphatases, which in turn dephosphorylate the transcription factors Rtg1/2 and Gln3 causing them to move into the nucleus and induce the expression of retrograde signalling genes and nitrogen discrimination genes, respectively. Once translocated into the nucleus, these transcription factors bind to specific DNA elements, alter the local chromatin state and recruit the general transcription machinery to mediate pre-initiation complex assembly and transcription by RNAPII. The exact mechanisms of these regulatory circuits are not fully understood but genome wide deletion screens in S.AbMole Clofentezine cerevisae have been a useful tool to identify novel factors that are required to mediate an efficient response to rapamycin. One of these factors is the peptidyl prolyl isomerase Rrd1. Rrd1D mutants exhibit multiple phenotypes including sensitivity to the carcinogen 4-nitroquinoline-1-oxide and UVA radiation, and, most prominently, extreme resistance to rapamycin. Rrd1 is evolutionally conserved and shares 35% identity with its human homologue PTPA. PTPA was first characterized to be an activator of the phospho-tyrosyl phosphatase activity of PP2A phosphatases in vitro. However, an in vivo role for this activity has not yet been described, and subsequent studies revealed that PTPA as well as Rrd1 are required for PP2A substrate specificity, complex formation and the reactivation of inactive PP2A complexes. Both were later found to possess intrinsic peptidyl prolyl isomerase activity on a specific PP2A peptide.