Construction of a Saccharomyces cerevisiae strain with a high level of RNA

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Breeding of yeast strains with higher RNA content is important because yeast RNA is a source of 5'-ribonucleotides used in food industries. To construct an S. cerevisiae strain with higher RNA content, we have employed a four-step breeding procedure. In the first step, an S. cerevisiae disruptant of the RRN5 or RRN10 gene, important components of rRNA transcription, was constructed. In the second step, suppressors were isolated that restored the slow growth of therrn10 disruptant. Since RRN5 is an essential gene, plasmid rescue method wasapplied in case of rrn5 disruptant. In the third step, wild-type RRN5 or RRN10 gene was introduced into each of the suppressors. In the final step, copy number of rDNA gene was increased from ca. 150 to 300 by genome engineering. By these strategies, we have successfully constructed S. cerevisiae strains producing 2-fold higher content of RNA compared with strains currently used in industry.