Cytological characterization of an Aspergillus nidulans mutant from a strain with chromosomic duplication

A development mutant, named V103, was obtained spontaneously from the A strain of A. nidulans. The A strain contains a duplicated segment of chromosome I that has undergone translocation to chromosome II (I ¨ II). It is mitotically unstable and generates phenotypically deteriorated types, some with enhanced stability. The deteriorated variants of A. nidulans show abnormal development, exhibiting slower colony growth, variations in colony pigmentation and changes in conidiophore structure. The alterations observed in the conidiophore include fewer metulae and phialides, further elongation and ramification of these structures, delayed nuclear migration and the presence of secondary conidiophores.

Occurrence of killer yeast strains in industrial and clinical yeast isolates

The secretion of proteinaceous toxins is a widespread characteristic in environmental and laboratory yeast isolates, a phenomenon called "killer system". The killer phenotype (K+) can be encoded by extrachromosomal genetic elements (EGEs) as double stranded DNA or RNA molecules (dsDNA, dsRNA) or in nuclear genes. The spectrum of action and the activity of killer toxins are influenced by temperature, salinity and pH of media. In the present work we determined the existence of K+ in a collection of S. cerevisiae and P. anómala yeasts isolated from environmental, industrial and clinical sources. The assays were performed in strains belonging to three yeast genera used as sensitive cells and under a wide range of pH and temperatures. Approximately 51 percent of isolates tested showed toxicity against at least one sensitive yeast strain under the conditions tested. The K+ P. anómala isolates showed a wide spectrum of action and two of them had toxic activity against strains of the three yeast genera assayed, including C. albicans strains. In all S. cerevisiae K+ isolates an extrachromosomal dsRNA molecule (4.2 Kb) was observed, contrary to P. anómala K+ isolates, which do not possess any EGEs. The K+ phenotype is produced by an exported protein factor and the kinetics of killer activity production was similar in all isolates with high activity in the log phase of growth, decaying in the stationary phase.

Mapping autonomously replicating sequence elements in a 73-kb region of chromosome II of the fission yeast, Schizosaccharomyces pombe.

Autonomously replicating sequence (ARS) elements are the genetic determinants of replication origin function in yeasts. They can be easily identified as the plasmids containing them transform yeast cells at a high frequency. As the first step towards identifying all potential replication origins in a 73-kb region of the long arm of fission yeast chromosome II, we have mapped five new ARS elements using systematic subcloning and transformation assay. 2D analysis of one of the ARS plasmids that showed highest transformation frequency localized the replication origin activity within the cloned genomic DNA. All the new ARS elements are localized in two clusters in centromere proximal 40 kb of the region. The presence of at least six ARS elements, including the previously reported ars727, is suggestive of a higher origin density in this region than that predicted earlier using a computer based search.

Chromosome instability in industrial strains of Saccharomyces cerevisiae batch cultivated under laboratory conditions

Industrial ethanol fermentation is a complex microbiological process to which yeast cells must adapt for survival. One of the mechanisms for adaptation is thought to involve chromosome rearrangements. We found that changes in chromosome banding patterns measured by pulsed-field gel electrophoresis can also be produced in laboratory media under simulated industrial conditions. Based on analysis of their generational variation, we found that these chromosome changes were specific to the genetic backgrounds of the initial strains. We conclude that chromosome rearrangements could be one of the factors involved in yeast cell adaptation to the industrial environment.

Study of a region on yeast chromosome XIII that complements pet G199 mutants (COX7) and carries a new non-essential gene

The mutants of Saccharomyces cerevisiae assigned to complementation group G199 are deficient in mitochondrial respiration and lack a functional cytochrome oxidase complex. Recombinant plasmids capable of restoring respiration were cloned by transformation of mutants of this group with a yeast genomic library. Sequencing indicated that a 2.1-kb subclone encompasses the very end (last 11 amino acids) of the PET111 gene, the COX7 gene and a new gene (YMR255W) of unknown function that potentially codes for a polypeptide of 188 amino acids (about 21.5 kDa) without significant homology to any known protein. We have shown that the respiratory defect corresponding to group G199 is complemented by plasmids carrying only the COX7 gene. The gene YMR255W was inactivated by one-step gene replacement and the disrupted strain was viable and unaffected in its ability to grow in a variety of different test media such as minimal or complete media using eight distinct carbon sources at three pH values and temperatures. Inactivation of this gene also did not affect mating or sporulation.

2-DG induced modulation of chromosomal DNA profile, cell survival, mutagenesis and gene conversion in X-irradiated yeast.

Effects of post-irradiation modulation in presence of 2-deoxy-D-glucose and yeast extract, on chromosomal DNA profile, cell survival, reverse mutation (ILV+) and gene conversion (TRP+), were studied in X-irradiated stationary phase yeast cells (diploid strain D7 of Saccharomyces cerevisiae). The damage and repair in chromosomal DNA bands, resolved by using contour clamped homogeneous electric pulsed-field gel electrophoresis (PFGE) technique, was estimated by calculating intensity ratio, rho n (rho n I(n)/I(t); where I(n) is the intensity of nth band in a lane and I(t) is the sum of intensities of all bands and the well in the lane). The data indicate linear correlation between relative compactness (tau) of a chromosome [chromosome size (Kb)/length of synaptonemal complex (microns)] and DNA damage and repair. The chromosome repair kinetics were biphasic, showing initial decrease followed by an increase in rho n. Variations were observed among different chromosomes with respect to DNA damage, repair and post-irradiation repair modulation. 2-DG inhibited both components of chromosomal DNA repair and also repair of potentially lethal damage but enhanced frequencies of mutants. Relatively the effects on revertants were greater in cells irradiated with lower doses (50 Gy) of X-rays and post-irradiation incubation in presence of phosphate buffer having 2-DG (50 mM) and glucose (10 mM). Yeast extract increased frequencies of revertants and convertants thus promoting error-prone DNA repair. Yeast extract in combination with 2-DG showed complex effects on chromosomal DNA repair and enhanced mutagenesis further.

Observaçöes citológicas em conídios e células miceliais de humicola sp / Cytological observations in conidial and mycelial cells of humicola sp

Resumo: O objetivo desse estudo foi descrever aspectos do comportamento nuclear durante a divisäo miótica em conídios e células miceliais do fungo celulolítico Humicola so. A sequência dos eventos mitóticos foi similar aquela observada em outros fungos filamentosos. O número de cromossomos observados no final da metáfase näo pode ser determinado com segurança, mas pareceu estar entre 6 e 8. As culturas de Humicola sp frequentemente mostraram anastomose de hifas e a presença de núcleos na ponte hifal, o que permitiria a ocorrência de heterocariose,e indica que o ciclo parasexual pode ocorrer nessa espécie (au)