Survival of genetically-engineered and wild-type strains of the yeast Saccharomyces cerevisiae under simulated environmental conditions: a contribution on risk assessment

Survival of genetically modified and self-cloned strains of commercial baker's yeast in simulated natural environments: environmental risk assessment

Although genetic engineering techniques for baker's yeast might improve the yeast's fermentation characteristics, the lack of scientific data on the survival of such strains in natural environments as well as the effects on human health prevent their commercial use. Disruption of acid trehalase gene (ATH1) improves freeze tolerance, which is a crucial characteristic in frozen-dough baking. In this study, ATH1 disruptants constructed by genetic modification (GM) and self-cloning (SC) techniques were used as models to study such effects because these strains have higher freeze tolerance and are expected to be used commercially. Behavior of the strains in simulated natural environments, namely, in soil and water, was studied by measuring the change in the number of viable cells and in the concentration of DNA that contains ATH1 loci. Measurements were made using a real-time PCR method during 40 days of cultivation. Results showed that the number of viable cells of GM and SC strains decreased in a time-dependent manner and that the decrease rate was nearly equal to or higher than that for wild-type (WT) yeast. For all three strains (SC, GM, and WT) in the two simulated natural environments (water and soil), the DNA remained longer than did viable cells but the decrease patterns of either the DNA or the viable cells of SC and GM strains had tendencies similar to those of the WT strain. In conclusion, disruption of ATH1 by genetic engineering apparently does not promote the survival of viable cells and DNA in natural environments.

A conditional suicide system forSaccharomyces cerevisiae relying on the intracellular production of theSerratia marcescens nuclease

A conditional lethal system for biological containment of genetically modified strains of Saccharomyces cerevisiae is described. This suicide system is based on the intracellular production of the Serratia marcescens nuclease in the yeast cell, aiming at the destruction of the host genetic material. The S. marcescens nuclease, encoded by the nucA gene, is normally secreted by the bacterium into the medium. In the present work, the nucA gene, devoid of its signal peptide coding sequence, was cloned in a yeast expression vector, under control of the glucose-repressed S. cerevisiae alcohol dehydrogenase 2 gene (ADH2) promoter. When transformed into S. cerevisiae, the recombinant plasmid proved to be effective in killing the host cells upon glucose depletion from the medium, and the nuclease activity was found in lysates prepared from the transformants. In addition, the nuclease degrading effect was shown to reach chromosomal DNA in the yeast host. The killing effect of the nucA plasmid was also demonstrated in soil microcosm assays, indicating that whenever the GMM escapes into the environment where glucose is scarce, the nucA gene will be expressed and the resulting nuclease will destroy the genetic material and kill the cells. In contrast to other suicide systems that target the cell envelope, the advantage of the one described here is that it disfavours horizontal gene transfer from recombinant yeast cells to other microorganisms found in the environment.

Fate of three genetically engineered, biotechnologically important microorganism species in soil: impact of soil properties and intraspecies competition with nonengineered strains

The fate of a bacterium and two yeast species genetically engineered by insertion of a nucleotide sequence encoding for aprotinin was studied in three different soils. Corynebacterium glutamicum carried the recombinant gene on plasmid pUN1, Saccharomyces cerevisiae carried the gene on plasmid p707, and in Pichia angusta (formerly Hansenula polymoropha) LR9-Apr8, the gene was chromosomally inserted with eight tandem repeats. Corynebacterium glutamicum persisted longer than both yeast strains. In a sandy loam of pH 5.9, recovery rates of cultured cells were lower than in a clay silt or a silty sand, with pH values of 7.1 and 6.7, respectively. Generally, persistence at 10 °C was higher than at 20 °C. An adaptation of the genetically engineered strains resulting in higher soil persistence was not observed for any of the three species tested. Competition experiments between nonengineered and genetically engineered strains in presterilized soils revealed a reduced fitness of the engineered strains. However, a more competitive C. glutamicum pUN1 evolved after reinoculation of cells, preselected by a preceding competition experiment.Key words: ecological risk assessment, genetic engineering, nondeliberate release, soil inoculation, aprotinin.

Direct detection of recombinant gene expression by two genetically engineered yeasts in soil on the transcriptional and translational levels

The expression of a recombinant gene by yeasts seeded into soil samples was directly measured by analyzing transcripts and gene product occurrences in soil extracts. Two yeast species, Saccharomyces cerevisiae WHL292 and Hansenula polymorpha LR9-Apr4, both engineered by a synthetic gene sequence encoding the mammalian peptide aprotinin, produced and secreted this peptide in batch cultures at concentrations of 90 and 64 ng ml-1, respectively. In S. cerevisiae, the aprotinin gene was located on plasmid p707 and expressed constitutively. H. polymorpha carried the gene chromosomally integrated, and its expression was inducible by methanol. To detect aprotinin transcripts, cells were directly lysed in the soil samples and the crude lysates were hybridized to oligo(dT)-coated magnetized polystyrene beads (Dynabeads). After separation and purification in a magnetic field, aprotinin mRNA was detected by reverse transcriptase PCR with aprotinin gene-specific primers. Transcripts from 10 cells g of soil-1 were sufficient for detection. When 10(7) cells of S. cerevisiae were inoculated into soil, aprotinin mRNA was detectable during the first 4 days. Addition of methanol and a combined nutrient solution was necessary to induce aprotinin gene expression of H. polymorpha in soil. Aprotinin could be detected directly in soil extracts by an indirect enzyme-linked immunosorbent assay with monoclonal aprotinin-specific antibodies. The detection threshold was 45 pg g of soil-1. In presterilized soil inoculated with S. cerevisiae (10(6) CFU g-1), aprotinin accumulated during the first 10 days to 12 ng g of soil-1 and then remained constant.(ABSTRACT TRUNCATED AT 250 WORDS)