Yeast (Saccharomyces cerevisiae) RNA is an important source of 5'-ribonucleotides that is used in both the food and pharmaceutical industries. Efficient transcription of rDNA is very important to construct yeast strains with high RNA content. The gene RRN10, which encodes, a component of the upstream activation factor, is essential to promote high-level transcription of rDNA. In our previous study, we isolated SupE strain as a dominant suppressor, which showed the ability to restore the severe growth defects and reduced RNA content caused by disruption of the RRN10 gene. SupE strain has multiple mutations which we designated collectively as SUPE. Further analysis on SUPE mutation indicated that RPL40A was responsible for suppression of defect caused by rrn10 disruption. However, there were no base changes in this gene as compared with the parental Delta rrn10 strain, thus suggesting that an additional copy of RPL40A suppress the defects caused by Delta rrn10 disruption, and that, in SupE strain, these defects are suppressed by increased transcription of RPL40A whose copy is doubled. When multiple copies of RPL40A were combined with SUPE mutation on an RRN10(+) background, the resultant SupE strain had significantly higher RNA content than wild-type strain. In addition, increased transcription of RP140B also showed significant effect to restore the growth defect and reduced RNA content caused by Delta rrn10 disruption. We propose a model to explain how SUPE mutation increases the transcription of ribosomal protein genes such as RPL40A and RPL40B in SupE strain, resulting in an increase in RNA content. (C) 2013, The Society for Biotechnology, Japan. All rights reserved.