Heather L. True

A yeast prion provides a mechanism for genetic variation and phenotypicdiversity

A major enigma in evolutionary biology is that new forms or functions often require the concerted effects of several independent genetic changes. It is unclear how such changes might accumulate when they are likely to be deleterious individually and be lost by selective pressure. The Saccharomyces cerevisiae prion [PSI+] is an epigenetic modifier of the fidelity of translation termination, but its impact on yeast biology has been unclear. Here we show that [PSI+] provides the means to uncover hidden genetic variation and produce new heritable phenotypes. Moreover, in each of the seven genetic backgrounds tested, the constellation of phenotypes produced was unique. We propose that the epigenetic and metastable nature of [PSI+] inheritance allows yeast cells to exploit pre-existing genetic variation to thrive in fluctuating environments. Further, the capacity of [PSI+] to convert previously neutral genetic variation to a non-neutral state may facilitate the evolution of new traits.

Epigenetic regulation of translation reveals hidden genetic variation to produce complex traits.

Phenotypic plasticity and the exposure of hidden genetic variation both affect the survival and evolution of new traits, but their contributing molecular mechanisms are largely unknown. A single factor, the yeast prion [PSI+], may exert a profound effect on both. [PSI+] is a conserved, protein-based genetic element that is formed by a change in the conformation and function of the translation termination factor Sup35p, and is transmitted from mother to progeny. Curing cells of [PSI+] alters their survival in different growth conditions and produces a spectrum of phenotypes in different genetic backgrounds. Here we show, by examining three plausible explanations for this phenotypic diversity, that all traits tested involved [PSI+]-mediated read-through of nonsense codons. Notably, the phenotypes analysed were genetically complex, and genetic re-assortment frequently converted [PSI+]-dependent phenotypes to stable traits that persisted in the absence of [PSI+]. Thus, [PSI+] provides a temporary survival advantage under diverse conditions, increasing the likelihood that new traits will become fixed by subsequent genetic change. As an epigenetic mechanism that globally affects the relationship between genotype and phenotype, [PSI+] expands the conceptual framework for phenotypic plasticity, provides a one-step mechanism for the acquisition of complex traits and affords a route to the genetic assimilation of initially transient epigenetic traits.

Ribonuclease P of Tetrahymena thermophila

Ribonuclease P (RNase P) is responsible for the generation of mature 5′ termini of tRNA. The RNA component of this complex encodes the enzymatic activity in bacteria and is itself catalytically active under appropriate conditions in vitro. The role of the subunits in eucaryotes has not yet been established. We have partially purified RNase P activity from the ciliate protozoan Tetrahymena thermophila to learn more about the biochemical characteristics of RNase P from a lower eucaryote. The Tetrahymena RNase P displays a pH optimum and temperature optimum characteristic of RNase P enzymes isolated from other organisms. The Km of the T. thermophila enzyme for pre-tRNAGln is 1.6 × 10−7 M, which is comparable to the values reported for other examples of RNase P. The Tetrahymena RNase P is a ribonucleoprotein complex, as supported by its sensitivity to micrococcal nuclease and proteinase K. The buoyant density of the enzyme in Cs2SO4 is 1.42 g/ml, which suggests that the RNA component of the Tetrahymena enzyme comprises a significantly greater percentage of the holoenzyme than that determined for RNase P of other Eucarya or Archaea. The holoenzyme has a requirement for divalent cations displaying characteristics that are unique for RNase P but closely resemble preferences reported for the Tetrahymena group I intron RNA. Puromycin inhibits pre-tRNA processing by the Tetrahymena complex, and implications of the similarities between recognition of tRNA by ribosomal components and RNase P are discussed.

Properties of β-lactamase from Pseudomonas syringae

Abstract.Pseudomonas syringae isolate BR2R produces tabtoxin, a β-lactam-containing antibiotic, and the causative agent of wildfire disease of green bean (Phaseolus vulgaris). β-Lactamase production has been suggested as the mechanism that protects P. syringae from tabtoxin. We sought to determine whether the organism produces β-lactamase and whether the enzyme plays a role in protection from this antibiotic. P. syringae and mutants defective in tabtoxin production and resistance produce β-lactamase. Three distinct β-lactamases with molecular weights of 41,000 were identified. The isoelectric points of the proteins were 6.1, 6.8, and 9.2. The enzymes preferentially hydrolyze cephalosporin. This investigation demonstrates that the organism produces multiple β-lactamases and describes characteristics of the proteins.