ABSTRACT
Nitrogen is an essential nutrient for yeast cells on ethanol fermentation. In order to reveal the promoting mechanisms of organic nitrogen sources on the ethanol fermentation by yeast, Saccharomyces cerevisiae, laser tweezers Raman spectroscopy and single-cell analysis techniques were used to monitored the kinetic of intracellular bio-macromolecules of individual cells during fermentation with urea, yeast extract, ammonium nitrate or ammonium sulfate as the sole nitrogen source. Major results from this work were as follows. (1) Organic nitrogen sources had a promoting effect on the ethanol fermentation, the fermentation with urea and yeast extract reached the maximum concentration of ethanol in 14-18 h. ( 2 ) There were no apparent lag phases for the RNA synthesis of yeast cells cultured with urea and yeast extract. The averaged Raman intensity of yeast cells at peak of 782 cm-1 in the early stage of fermentation was stronger than that of cultured with ammonium nitrate and ammonium sulfate. The maximum was about 1. 9-2. 1 times of the initial intensity for urea or yeast extract, but 1. 2-1. 4 times for ammonium nitrate and ammonium sulfate. (3) The secondary structure of proteins of partial cells cultured with yeast extract was dominated byβ-sheet, while cells cultured with other nitrogen sources were dominated by α-helix absolutely. These results bring us the conclusion that the improving effect of organic nitrogen sources such as urea and yeast extract on ethanol fermentation by Saccharomyces cerevisiae may be due to that the organic nitrogen sources can shorten the lag phase of yeast cells, promote the RNA synthesis, and promote the transcription and expression of related genes.