Bianca Maria Ranzi

Glycerol production in a triose phosphate isomerase deficient mutant of Saccharomyces cerevisiae.

Interesting challenges from metabolically engineered Saccharomyces cerevisiae cells arise from the opportunity to obtain yeast strains useful for the production of chemicals. In this paper, we show that engineered yeast cells deficient in the triose phosphate isomerase activity are able to produce glycerol without the use of steering agents. High yields of conversion of glucose into glycerol (80−90% of the theoretical yield) and productivity (1.5 g L−1 h−1) have been obtained by a bioconversion process carried out in a poor and clean medium. We obtained indications that the growth phase at which the biomass was collected affect the process. The best results were obtained using cells collected at the end of exponential phase of growth. In perspective, the strategies and the information about the physiology of the cells described here could be useful for the developing of new biotechnological processes for glycerol production, outflanking the problems related to the use of high level of steering agents.

Alterations of the glucose metabolism in a triose phosphate isomerase-negative Saccharomyces cerevisiae mutant

The absence of triose phosphate isomerase activity causes an accumulation of only one of the two trioses, dihydroxyacetone phosphate, and this produces a shift in the final product of glucose catabolism from ethanol to glycerol (Compagno et al., 1996 ). Alterations of glucose metabolism imposed by the deletion of the TPI1 gene in Saccharomyces cerevisiae were studied in batch and continuous cultures. The Δtpi1 null mutant was unable to grow on glucose as the sole carbon source. The addition of ethanol or acetate in media containing glucose, but also raffinose or galactose, relieved this effect in batch cultivation, suggesting that the Crabtree effect is not the primary cause for the mutants impaired growth on glucose. The addition of an energy source like formic acid restored glucose utilization, suggesting that a NADH/energy shortage in the Δtpi1 mutant could be a cause of the impaired growth on glucose. The amount of glycerol production in the Δtpi1 mutant could represent a good indicator of the fraction of carbon source channelled through glycolysis. Data obtained in continuous cultures on mixed substrates indicated that different contributions of glycolysis and gluconeogenesis, as well as of the HMP pathway, to glucose utilization by the Δtpi1 mutant may occur in relation to the fraction of ethanol present in the media. Copyright

Improved Secretion of Native Human Insulin-Like Growth Factor 1 from gas1 Mutant Saccharomyces cerevisiae Cells

ABSTRACT We studied the secretion of recombinant human insulin-like growth factor 1 (rhIGF-1) from transformed yeast cells. The hIGF-1gene was fused to the mating factor α prepro- leader sequence under the control of the constitutive ACT1 promoter. We found that the inactivation of the GAS1 gene in the host strain led to a supersecretory phenotype yielding a considerable increase, from 8 to 55 mg/liter, in rhIGF-1 production.

Decarboxylation of cinnamic acids by Saccharomyces cerevisiae

Abstract The stereochemistry of the decarboxylation of 3,4-dimethoxycinnamic acids by Saccharomyces cerevisiae and the enzymatic specificity with respect to the substrate structure were studied. This reaction proceeds with retention of configuration at the side-chain double bond as well as enzymatic specificity for the (E) configuration. The influence of substituents in the α and β positions was also studied. Hypotheses on the reaction mechanism were proposed.

Aerobic sugar metabolism in the spoilage yeast Zygosaccharomyces bailii

Despite the importance of some Zygosaccharomyces species as agents causing spoilage of food, the carbon and energy metabolism of most of them is yet largely unknown. This is the case with Zygosaccharomyces bailii. In this study the occurrence of the Crabtree effect in the petite-negative yeast Z. bailii ATCC 36947 was investigated. In this yeast the aerobic ethanol production is strictly dependent on the carbon source utilised. In glucose-limited continuous cultures a very low level of ethanol was produced. In fructose-limited continuous cultures ethanol was produced at a higher level and its production increased with the dilution rate. As a consequence, on fructose the onset of respiro-fermentative metabolism caused a reduction in biomass yield. An immediate aerobic alcoholic fermentation in Z. bailii was observed during the transition from sugar limitation to sugar excess, both on glucose and on fructose. The analysis of some key enzymes of the fermentative metabolism showed a high level of acetyl-CoA synthetase in Z. bailii growing on fructose. At high dilution rates, the activities of glucose- and fructose-phosphorylating enzymes, as well as of pyruvate decarboxylase and alcohol dehydrogenase, were higher in cells during growth on fructose than on glucose.

Fermentation of whey and starch by transformed Saccharomyces cerevisiae cells.

Among the main agro-industrial wastes, whey and starch are of prime importance. In previous work we showed that strains ofSaccharomyces cerevisiae transformed with the episomal plasmid pM1 allow production of yeast biomass and ethanol from whey/lactose. Ethanol production from whey and derivatives has been improved in computer-controlled bioreactors, while fermentation studies showed that the composition of the medium greatly modulates the productivity (g ethanol produced.l in 1 h of fermentation). A yeast strain for the simultaneous utilization of lactose and starch has also been developed. Biotechnological perspective are discussed.

Development of high cell density cultures of engineered Saccharomyces cerevisiae cells able to grow on lactose

Saccharomyces cerevisiae cells transformed with a multicopy expression vector bearing an in frameSTA2-LacZ gene fusion under the control of the galactose inducibleUASGAL/CYC1 promoter, secrete β-galactosidase in the periplasmic space. Fermentation studies showed that with this transformed strain it is possible to reach on lactose high cell densities with both high productivity and high growth yield by means of fed-batch fermentations.

Identification of different daughter and parent subpopulations in an asynchronously growing Saccharomyces cerevisiae population

Under all growth conditions, a growing Saccharomyces cerevisiae yeast population is extremely heterogeneous, since individual cells differ in their cell size; this is due to their position in the cell division cycle and their genealogical age. To gain insight into the structure of a growing yeast population, we used a recently developed flow cytometric approach which enables, in asynchronously growing S. cerevisiae populations, tagging of both the cell age and the protein content of individual cells. This approach enabled the identification of daughter cells belonging to different cell cycle positions (i.e. newborn, G1, S + G2 + M + G1*, and dividing), thus yielding information about the relative fraction in the whole population, cell size and variability. More limited information could be obtained for the parent subpopulation; however, we were able to identify and characterize the dividing parent cells. The coefficient of variation (CV) of the protein content distribution for the dividing parents (27) was much higher than the CV of dividing daughters (18). Further findings obtained indicated a large overlap between the cell protein content distributions of daughter and parent cells as well as between the protein content of cells of the same subpopulation but belonging to different stages of the cell division cycle. The analysis of these differences enables a better understanding of the complex structure of an asynchronously growing yeast population.

DEVELOPMENT OF A PH-CONTROLLED FED-BATCH SYSTEM FOR BUDDING YEAST

In the manufacturing of bakers yeast by aerobic fed-batch systems, continuous assessment of the state of the process is necessary for regulating the flow rate (on/off) for growth medium addition. A new, simple method for the fed-batch yeast process has been developed. It is based on pH changes as a suitable parameter for regulating the feed of fresh concentrated medium in response to metabolic activities of the yeast population. Experimental results have shown that it enables the attaining of high cell density with both high productivity and high yields.

Reduction of cinnamyl alcohols and cinnamaldehydes by Saccharomyces cerevisiae

Abstract The reduction of 3′,4′-dimethoxycinnamyl alcohols to phenylpropanols by Saccharomyces cerevisiae proceeds through the corresponding aldehydes. The specificity with respect to substrate structure of the two enzymatic systems involved in the above transformation (alcohol dehydrogenase and reductase) was studied. Whereas yeast alcohol dehydrogenase shows specificity for the ( E ) configuration of side-chain double bond, reductase does not act on 3-substituted substrates.