| Summary: | <p>Abstract</p> <p>Background</p> <p>In the fission yeast <it>Schizosaccharomyces pombe</it>, the <it>phx1</it><sup>+</sup> (pombe homeobox) gene was initially isolated as a multi-copy suppressor of lysine auxotrophy caused by depletion of copper/zinc-containing superoxide dismutase (CuZn-SOD). Overproduction of Phx1 increased the synthesis of homocitrate synthase, the first enzyme in lysine biosynthetic pathway, which is labile to oxidative stress. Phx1 has a well conserved DNA-binding domain called homeodomain at the N-terminal region and is predicted to be a transcription factor in <it>S. pombe</it>. However, its role has not been revealed in further detail. Here we examined its expression pattern and the phenotype of its null mutant to get clues on its function.</p> <p>Results</p> <p>Fluorescence from the Phx1-GFP expressed from a chromosomal fusion gene demonstrated that it is localized primarily in the nucleus, and is distinctly visible during the stationary phase. When we replaced the N-terminal homeobox domain of Phx1 with the DNA binding domain of Pap1, a well-characterized transcription factor, the chimeric protein caused the elevation of transcripts from Pap1-dependent genes such as <it>ctt1</it><sup><it>+</it></sup> and <it>trr1</it><sup><it>+</it></sup>, suggesting that Phx1 possesses transcriptional activating activity when bound to DNA. The amount of <it>phx1</it><sup><it>+</it></sup> transcripts sharply increased as cells entered the stationary phase and was maintained at high level throughout the stationary phase. Nutrient shift down to low nitrogen or carbon sources caused <it>phx1</it><sup><it>+</it></sup> induction during the exponential phase, suggesting that cells need Phx1 for maintenance function during nutrient starvation. The <it>Δphx1</it> null mutant showed decreased viability in long-term culture, whereas overproduction of Phx1 increased viability. Decrease in long-term survival was also observed for <it>Δphx1</it> under N- or C-starved conditions. In addition, <it>Δphx1</it> mutant was more sensitive to various oxidants and heat shock. When we examined sporulation of the <it>Δphx1/Δphx1</it> diploid strain, significant decrease in the formation of meiotic spores was observed.</p> <p>Conclusions</p> <p>Phx1 is a transcriptional regulator whose synthesis is elevated during stationary phase and by nutrient starvation in <it>S. pombe</it>. It supports long-term survival and stress tolerance against oxidation and heat, and plays a key role in the formation of meiotic spores.</p>
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