Ebook Current topics in medical mycology (Vol.4): Part 2
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Ebook Current topics in medical mycology (Vol.4): Part 2
5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2concept of yeasts has changed vastly. Once thought of as “£. coll with a nucleus,”1 these organisms currently represent cells of universal use by modern molecular biologists. Retroviral elements, ubiquitin, calmodulin, actin, and tubulin are only some of the many biological elements that are being i Ebook Current topics in medical mycology (Vol.4): Part 2nvestigated in yeast cells. Ras-related genes, strongly implicated in the transformation of normal mammalian to cancer cells,2 have also been discoverEbook Current topics in medical mycology (Vol.4): Part 2
ed in yeasts, opening the way for exciting new cytological, biochemical, and experimental genetic strategies that were impossible to carry' out in ani5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2ifferent fungal species. In most of these species, the persistent viral infection produces no discernible effect on the fungal host’s phenotype.However, the finding that the antiviral and interfcron-inducing activities of extracts (statolon, hellenin) from Penicillium species were due to the presenc Ebook Current topics in medical mycology (Vol.4): Part 2e of double-stranded RNA (dsRNA) has sparked an explosion of interest in what has now become a new area for research in mycology.4 The hypovirulence oEbook Current topics in medical mycology (Vol.4): Part 2
f certain forms of Endothia parasitica, which may cause chestnut blight disease, has also been found to be related to the presence of lipid-rich cytop5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2ked to specific dsRNA segments in Ophiostoma ulmi (which causes Dutch elm disease).The biological properties of the dsRNA genomes that are in capsid form in noninfectious VLPs within the fungal cytoplasm have proved to be unique. These mycoviruses are apparently incapable of extracellular transmissi Ebook Current topics in medical mycology (Vol.4): Part 2on through lysis of the host cell. They are normally maintained at a relatively stable copy number and transmitted predominantly by the intracellularEbook Current topics in medical mycology (Vol.4): Part 2
route. In hyphomycetes, for example, transmission occurs through the flow137138Polonelli et al.of protoplasm toward the growing hyphal tip, whereas cy5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2unusual characteristic of the dsRNA VLPs in fungi is that they generally produce no adverse effect on their hosts and, in some cases, may even be beneficial. In this respect they may be compared with bacterial plasmids, in that they act as nonessential extrachromosomal elements which can, however, b Ebook Current topics in medical mycology (Vol.4): Part 2e quite useful to their hosts.In addition to conjugation behavior, enterotoxin production, and antibiotic resistance, bacterial plasmids encode the prEbook Current topics in medical mycology (Vol.4): Part 2
oduction of bacteriocins by certain strains that exert lethal effects on closely related strains. The killer toxins produced by certain yeasts are ext5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2ctors. The resemblance is even more striking between killer yeasts and Paramecium species carrying K particles. In the latter case, a dominant nuclear gene has been found to be responsible for the organism’s ability to maintain these particles.5 The occurrence of similar phenomena in organisms with Ebook Current topics in medical mycology (Vol.4): Part 2such widely divergent evolutionary histories is fascinating.The Killer Phenomenon in YeastThe yeast killer phenomenon was first observed among SaccharEbook Current topics in medical mycology (Vol.4): Part 2
omyces cere-visiae strains.6 These strains secrete glycoproteins that have toxic effects on other, sensitive strains of this species as well as closel5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2her species are all encoded by satellite dsRNAs.In s. cerevisiae there are two main recognized killer systems (KI and K2) which do not elicit cross-immunity. KI is found in laboratory strains and is the most deeply investigated yeast killer system, whereas K2 is exclusively related to wine yeasts, w Ebook Current topics in medical mycology (Vol.4): Part 2hen no distinction is made, reference to the KI killer system is generally intended. In the type I system, cells may have one of four possible phenotyEbook Current topics in medical mycology (Vol.4): Part 2
pes: K+ R+ (killer), K“ R“ (sensitive), K“ R+ (neutral), and K+ R- (suicidal).Two types of dsRNA have been found inside icosahedral VLPs of approximat5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2he M dsRNA appears in about 100 copies. Deletion of certain sequences of the latter results in suppressive s dsRNA.7 Both L and M dsRNA interact with mak (maintenance killer) genes, which are widely scattered over the chromosomal map of the yeast, and these genes are necessary for maintaining autore Ebook Current topics in medical mycology (Vol.4): Part 2plication of M dsRNA VLPs. In fact, mutants that have5—Killer System Interactions139lost certain mak genes also lose M dsRNA, though L dsRNA, which isEbook Current topics in medical mycology (Vol.4): Part 2
also present in nonkiller yeast cells, is maintained. Recessive mutations of other chromosomal genes, such as ski (superkiller), result in increased 5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2secretion (sec) and killer expression (kex) of the toxin as well as for the virus-mediated host immunity to the toxin (vpl [vacuolar protein localization], end [endocytosis], and rex [resistance expression] genes) although not necessarily for replication of the cytoplasmic killer genome. The kex 2 g Ebook Current topics in medical mycology (Vol.4): Part 2ene product is, however, also required for mating functions and meiotic sporulation.The relationship between the L and M dsRNA genomes would be analogEbook Current topics in medical mycology (Vol.4): Part 2
ous to that between a helper and a defective virus rather than that of components of an interdependent segmented mycovirus genome.9 In the s. cerevisi5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2me capsid, and although the polypeptide composition of the VLPs has not been completely described yet, particles containing L dsRNA reportedly include a large polypeptide of 75,000 Da and two smaller ones of 55,000 and 37,000 Da. whether or not the M dsRNA-containing particles are structurally ident Ebook Current topics in medical mycology (Vol.4): Part 2ical to the L particles cannot be confirmed at this time. However, in light of the cross-antigenicity observed between the two VI,Ps, it is likely thaEbook Current topics in medical mycology (Vol.4): Part 2
t they share at least one polypeptide.The M dsRNA is related to toxin activity', by encoding the killer as well as the immune phenotype. Killer toxin-5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2es a 35-kDa protein (preprotoxin), which contains a signal sequence-encoding region. Either this molecule or a processed product (43-kDa protoxin) of glycosylation in the endoplasmic reticulum is responsible for immunity of the toxin-producing strain. After digestion by specific proteolytic enzymes, Ebook Current topics in medical mycology (Vol.4): Part 2 the protoxin is processed in the Golgi apparatus or in secretory vesicles into the mature killer toxin, which in the s. cerevisiae KI killer system iEbook Current topics in medical mycology (Vol.4): Part 2
s composed of two disulfide-linked a (9.5-kDa) and /3 (9.0-kDa) polypeptide components.1213Mutation of various nuclear genes may drastically affect th5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2y which killer toxins destroy sensitive yeast cells. Strains of s. cerevisiae that had been rendered resistant to s. cerevisiae KI killer toxin by mutation of the nuclear genes krel and kre2 were found to bind ^S-labeled killer toxin more weakly than wild, sensitive strains.14 Although killing activ Ebook Current topics in medical mycology (Vol.4): Part 2ity is not yet fully understood, we do know that rapid, energy-independent binding of the toxin to a (l,6)-/3-D-glucan-linked component of the cell waEbook Current topics in medical mycology (Vol.4): Part 2
ll occurs during the initial phase and that the products of either the krel or the kre2 nuclear gene are necessary' to this process. Mutations in the 5----Killer System InteractionsL. Polonelli, G. Morace, s. Conti, M. Gerloni, w. Magliani and c. ChezziViruses and FungiOver the last few years, our c Ebook Current topics in medical mycology (Vol.4): Part 2to the wall, presumptively by the /3 subunit, might make the a subunit of the toxin somehow accessible by an energydependent process at a plasma membrane site where the toxic effect is manifested by ion leakage and cell death.15 when constant concentrations of sensitive cells were treated with subsa Ebook Current topics in medical mycology (Vol.4): Part 2turating concentrations of killer toxin, linear rates of killing were observed, thus suggesting a single-hit process.16 Since krel and kre2 mutant sphEbook Current topics in medical mycology (Vol.4): Part 2
eroplasts are sensitive to the toxin, while those with mutation of the kre3 nuclear gene are resistant, even though normal cell wall binding occurs, iGọi ngay
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