Neutral lipid accumulation at elevated temperature in conditional mutants of two microalgae species.

Triacylglycerols, an energy storage compound in microalgae, are known to be accumulated after nitrogen starvation of microalgae cells. Microalgae could be of importance for future biodiesel production due to their fast growth rate and high oil content. In collections of temperature sensitive mutants of Chlamydomonas reinhardtii and Chlorella vulgaris, nine out of fourty-one mutants in C. reinhardtii and eleven out of fifty-three mutants in C. vulgaris contained increased amounts of neutral lipids, predominantly as triacylglycerols. Upon temperature induced cell-cycle arrest, these mutants showed enlarged cellular volume compared with the wild type. The C. reinhardtii mutants were analyzed further and one type of mutants displayed a shift in lipid composition from polar membrane lipids to neutral lipids after a temperature up-shift, while the second type of mutants accumulated more total lipid per cell, predominantly as neutral lipids as compared with the wild type. Three C. reinhardtii mutants were analyzed further and found to be arrested after DNA synthesis but prior to cell division in the cell cycle. These mutants will be useful in order to further understand neutral lipid accumulation in microalgae and suggest possibilities for biodiesel production by specific induction of lipid accumulation in miroalgal cultures by cell-cycle inhibition.

The dynamics of the Saccharomyces carlsbergensis brewing yeast transcriptome during a production-scale lager beer fermentation.

The transcriptome of a lager brewing yeast (Saccharomyces carlsbergensis, syn. of S. pastorianus), was analysed at 12 different time points spanning a production-scale lager beer fermentation. Generally, the average expression rapidly increased and had a maximum value on day 2, then decreased as the sugar got consumed. Especially genes involved in protein and lipid biosynthesis or glycolysis were highly expressed during the beginning of the fermentation. Similarities as well as significant differences in expression profiles could be observed when comparing to a previous transcriptome analysis of a laboratory yeast grown in YPD. The regional distribution of various expression levels on the chromosomes appeared to be random or near-random and no reduction in expression near telomeres was observed.

The level of MXR1 gene expression in brewing yeast during beer fermentation is a major determinant for the concentration of dimethyl sulfide in beer.

DMS (dimethyl sulfide) is an important beer flavor compound which is derived either from the beer wort production process or via the brewing yeast metabolism. We investigated the contribution of yeast MXR1 gene activity to the final beer DMS content. The MXR1-CA gene from Saccharomyces carlsbergensis (synonym of Saccharomyces pastorianus) lager brewing yeast was isolated and sequenced, and found to be 88% identical with Saccharomyces cerevisiae MXR1. Inactive deletion alleles of both genes were substituted for their functional counterparts in S. carlsbergensis. Such yeasts fermented well and did not form DMS from dimethyl sulfoxide. Overexpression in brewing yeast of MXR1 from non-native promoters with various strengths and transcription profiles resulted in an enhanced and correlated DMS production. The promoters of MXR1 and MXR1-CA contain conserved Met31p/Met32p binding sites, and in accordance with this were found to be co-regulated with the genes of the sulfur assimilation pathway. In addition, conserved YRE-like DNA sequences are present in these promoters, indicating that Yap1p may also take part in the control of these genes.