Growth at near-zero specific growth rates is a largely unexplored area of yeast physiology. To investigate the physiology of Saccharomyces cerevisiae under these conditions, the effluent removal pipe of anaerobic, glucoselimited chemostat culture (dilution rate, 0.025 h ؊1 ) was fitted with a 0.22-m-pore-size polypropylene filter unit. This setup enabled prolonged cultivation with complete cell retention. After 22 days of cultivation, specific growth rates had decreased below 0.001 h ؊1 (doubling time of >700 h). Over this period, viability of the retentostat cultures decreased to ca. 80%. The viable biomass concentration in the retentostats could be accurately predicted by a maintenance coefficient of 0.50 mmol of glucose g ؊1 of biomass h ؊1 calculated from anaerobic, glucose-limited chemostat cultures grown at dilution rates of 0.025 to 0.20 h ؊1 . This indicated that, in contrast to the situation in several prokaryotes, maintenance energy requirements in S. cerevisiae do not substantially change at near-zero specific growth rates. After 22 days of retentostat cultivation, glucose metabolism was predominantly geared toward alcoholic fermentation to meet maintenance energy requirements. The strict correlation between glycerol production and biomass formation observed at higher specific growth rates was not maintained at the near-zero growth rates reached in the retentostat cultures. In addition to glycerol, the organic acids acetate, D-lactate, and succinate were produced at low rates during prolonged retentostat cultivation. This study identifies robustness and by-product formation as key issues in attempts to uncouple growth and product formation in S. cerevisiae.Laboratory studies on microorganisms are often performed in batch cultures. During the initial phase of batch cultivation, all nutrients are usually present in excess. As a consequence, the initial specific growth rate, , of the microorganism in such cultures equals the maximum specific growth rate, max . In natural environments, the specific growth rate of microorganisms is likely to be constrained by the limited availability of one or more growth-limiting nutrients, resulting in specific growth rates far below max (8,24). In chemostat cultures fed with a medium containing a single growth-limiting nutrient, the dilution rate determines the specific growth rate. Chemostat cultivation therefore offers the possibility to study microbial physiology at carefully controlled, submaximal specific growth rates and to investigate the effect of specific growth rate on cellular physiology (20). Chemostat cultivation of the yeast Saccharomyces cerevisiae has demonstrated strong effects of specific growth rate on biomass composition (26, 51), product formation (5, 37), and cell size (23). Moreover, during energy-limited growth at low specific growth rates, a relatively large fraction of the energy substrate has to be dissimilated for maintenancerelated processes such as maintenance of chemi-osmotic gradients and turnover of cellular components (34). Not surprisi...

Extremely low specific growth rates (below 0.01 h−1) represent a largely unexplored area of microbial physiology. In this study, anaerobic, glucose-limited retentostats were used to analyse physiological and genome-wide transcriptional responses of Saccharomyces cerevisiae to cultivation at near-zero specific growth rates. While quiescence is typically investigated as a result of carbon starvation, cells in retentostat are fed by small, but continuous carbon and energy supply. Yeast cells cultivated near-zero specific growth rates, while metabolically active, exhibited characteristics previously associated with quiescence, including accumulation of storage polymers and an increased expression of genes involved in exit from the cell cycle into G0. Unexpectedly, analysis of transcriptome data from retentostat and chemostat cultures showed, as specific growth rate was decreased, that quiescence-related transcriptional responses were already set in at specific growth rates above 0.025 h−1. These observations stress the need for systematic dissection of physiological responses to slow growth, quiescence, ageing and starvation and indicate that controlled cultivation systems such as retentostats can contribute to this goal. Furthermore, cells in retentostat do not (or hardly) divide while remaining metabolically active, which emulates the physiological status of metazoan post-mitotic cells. We propose retentostat as a powerful cultivation tool to investigate chronological ageing-related processes.

Increasing protein expression levels is a key step in the commercial production of enzymes. Predicting promoter activity and translation initiation efficiency based solely on consensus sequences have so far met with mixed results. Here, we addressed this challenge using a "brute-force" approach by designing and synthesizing a large combinatorial library comprising ∼12 000 unique synthetic expression modules (SEMs) for Bacillus subtilis. Using GFP fluorescence as a reporter of gene expression, we obtained a dynamic expression range that spanned 5 orders of magnitude, as well as a maximal 13-fold increase in expression compared with that of the already strong veg expression module. Analyses of the synthetic modules indicated that sequences at the 5'-end of the mRNA were the most important contributing factor to the differences in expression levels, presumably by preventing formation of strong secondary mRNA structures that affect translation initiation. When the gfp coding region was replaced by the coding region of the xynA gene, encoding the industrially relevant B. subtilis xylanase enzyme, only a 3-fold improvement in xylanase production was observed. Moreover, the correlation between GFP and xylanase expression levels was weak. This suggests that the differences in expression levels between the gfp and xynA constructs were due to differences in 5'-end mRNA folding and consequential differences in the rates of translation initiation. Our data show that the use of large libraries of SEMs, in combination with high-throughput technologies, is a powerful approach to improve the production of a specific protein, but that the outcome cannot necessarily be extrapolated to other proteins.

Cultivation methods used to investigate microbial calorie restriction often result in carbon and energy starvation. This study aims to dissect cellular responses to calorie restriction and starvation in Saccharomyces cerevisiae by using retentostat cultivation. In retentostats, cells are continuously supplied with a small, constant carbon and energy supply, sufficient for maintenance of cellular viability and integrity but insufficient for growth. When glucose-limited retentostats cultivated under extreme calorie restriction were subjected to glucose starvation, calorie-restricted and glucose-starved cells were found to share characteristics such as increased heat-shock tolerance and expression of quiescence-related genes. However, they also displayed strikingly different features. While calorie-restricted yeast cultures remained metabolically active and viable for prolonged periods of time, glucose starvation resulted in rapid consumption of reserve carbohydrates, population heterogeneity due to appearance of senescent cells and, ultimately, loss of viability. Moreover, during starvation, calculated rates of ATP synthesis from reserve carbohydrates were 2-3 orders of magnitude lower than steady-state ATP-turnover rates calculated under extreme calorie restriction in retentostats. Stringent reduction of ATP turnover during glucose starvation was accompanied by a strong down-regulation of genes involved in protein synthesis. These results demonstrate that extreme calorie restriction and carbon starvation represent different physiological states in S. cerevisiae.

The y-axis labels "q lactate (mmol ⅐ g Ϫ1 ⅐ h Ϫ1)" and "q acetate , q succinate (mmol ⅐ g Ϫ1 ⅐ h Ϫ1)" should read "q lactate (mol ⅐ g Ϫ1 ⅐ h Ϫ1)" and "q acetate , q succinate (mol ⅐ g Ϫ1 ⅐ h Ϫ1)," respectively.

Engineering Saccharomyces cerevisiae for the utilization of pentose sugars is an important goal for the production of second-generation bioethanol and biochemicals. However, S. cerevisiae lacks specific pentose transporters, and in the presence of glucose, pentoses enter the cell inefficiently via endogenous hexose transporters (HXTs). By means of in vivo engineering, we have developed a quadruple hexokinase deletion mutant of S. cerevisiae that evolved into a strain that efficiently utilizes D-xylose in the presence of high D-glucose concentrations. A genome sequence analysis revealed a mutation (Y353C) in the general corepressor CYC8, or SSN6, which was found to be responsible for the phenotype when introduced individually in the nonevolved strain. A transcriptome analysis revealed altered expression of 95 genes in total, including genes involved in (i) hexose transport, (ii) maltose metabolism, (iii) cell wall function (mannoprotein family), and (iv) unknown functions (seripauperin multigene family). Of the 18 known HXTs, genes for 9 were upregulated, especially the low or nonexpressed HXT10, HXT13, HXT15, and HXT16. Mutant cells showed increased uptake rates of D-xylose in the presence of D-glucose, as well as elevated maximum rates of metabolism (V max ) for both D-glucose and D-xylose transport. The data suggest that the increased expression of multiple hexose transporters renders D-xylose metabolism less sensitive to D-glucose inhibition due to an elevated transport rate of D-xylose into the cell. IMPORTANCEThe yeast Saccharomyces cerevisiae is used for second-generation bioethanol formation. However, growth on xylose is limited by pentose transport through the endogenous hexose transporters (HXTs), as uptake is outcompeted by the preferred substrate, glucose. Mutant strains were obtained with improved growth characteristics on xylose in the presence of glucose, and the mutations mapped to the regulator Cyc8. The inactivation of Cyc8 caused increased expression of HXTs, thereby providing more capacity for the transport of xylose, presenting a further step toward a more robust process of industrial fermentation of lignocellulosic biomass using yeast.KEYWORDS sugar transporter, xylose transport, evolutionary engineering, transcriptome, yeast A n increasing energy demand and concerns of obtaining this energy from fossil fuels have stimulated the development of liquid fuels from renewable feedstock. Bioethanol, mostly used as a fuel additive, produced from readily fermentable agricultural feedstocks, such as sugar cane and corn, is less desired because the production of these feedstocks requires large amounts of arable land and competes with the food supply (1). A more sustainable source of feedstock is lignocellulosic biomass from hardwood, softwood, and agricultural residues (2). However, a major drawback of lignocellulosic feedstocks is the inability of the most commonly used yeast in industry,