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Encyclopedia > PSI (prion)

[PSI+] was first identified by Cox as a modifier of some types of translational suppression of mutant genes. The initial discovery of [PSI+] was made in a strain auxotrophic for the amino acid adenine due to a nonsense mutation [1] Further investigation found that [PSI+] is the misfolded form of Sup35, which is an important factor for translation termination during protein synthesis [2]. It is believed that [PSI+] causes suppression of nonsense mutations by sequestering functional Sup35 in non-functional aggregates, thereby allowing stop codon readthrough. [PIN+], in turn, is the misfolded form of the protein Rnq1. However, the normal function of this protein is unknown to date. It is of note that for the induction of most variants of [PSI+], the presence of [PIN+] is required. Though reasons for this are poorly understood, it is suggested that [PIN+] aggregates may act as “seeds” for the polymerization of [PSI+] [3]. Two modified versions of Sup35 have been created that can induce [PSI+] in the absence of [PIN+] when overexpressed. One version was created by digestion of the gene with BalI, which results in a protein consisting of only the M and N portions of Sup35 [4]. The other is a fusion of Sup35NM with HPR, a human membrane receptor protein. Wikipedia does not have an article with this exact name. ... Fungal prions have been investigated and lead to a deeper understanding of disease-forming mammilian prions. ... For other meanings of this term, see gene (disambiguation). ... Phenylalanine is one of the standard amino acids. ... Adenine is one of the two purine nucleobases used in forming nucleotides of the nucleic acids DNA and RNA. In DNA, adenine binds to thymine via two hydrogen bonds to assist in stabilizing the nucleic acid structures. ... Biological and artificial methods for creation of proteins differ significantly. ...

Laboratories commonly identify [PSI+] by growth of a strain auxotrophic for adenine on media lacking adenine, similar to that used by Cox et al. These strains cannot synthesize adenine due to a nonsense mutation in one of the enzymes involved in biosynthetic pathway. When the strain is grown on yeast-extract/dextrose/peptone media (YPD), the blocked pathway results in buildup of a red-colored intermediate compound, which is exported from the cell due to its toxicity. Hence, color is an alternative method of identifying [PSI+] -- [PSI+] strains are white or pinkish in color, and [psi-] strains are red. A third method of identifying [PSI+] is by the presence of Sup35 in the pelleted fraction of cellular lysate.


  1. ^ Cox, B. S., M. F. Tuite and C. S. McLaughlin (1988). "The [PSI+]-Factor of Yeast - a Problem in Inheritance". Yeast 4, 159–178
  2. ^ Paushkin, S. V., V. V. Kushnirov, V. N. Smirnov and M. D. Ter-Avanesyan (1996). "Propagation of the yeast prion-like [PSI+] determinant is mediated by oligomerization of the SUP35-encoded polypeptide chain release factor". EMBO (European Molecular Biology Organization) Journal 15, 3127–3134
  3. ^ Chernoff, Y. O. (2001). "Mutation processes at the protein level: Is Lamarck back?". Mutation Research 488, 39–64
  4. ^ Derkatch, I. L., M. E. Bradley, P. Zhou, Y. O. Chernoff and S. W. Liebman (1997). "Genetic and Environmental Factors Affecting the de novo Appearance of the [PSI+] Prion in Saccharomyces cerevisiae". Genetics 147 507–519



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