Autophagy-deficient Schizosaccharomyces pombe mutants undergo partial sporulation during nitrogen starvation

Autophagy is triggered when organisms sense radical environmental changes, including nutritional starvation. During autophagy, cytoplasmic components, including organelles, are enclosed within autophagosomes and are degraded upon lysosome-vacuole fusion. In this study, we show that processing of GFP-tagged Atg8 can serve as a marker for autophagy in the fission yeast Schizosaccharomyces pombe. Using this marker, 13 Atg homologues were also found to be required for autophagy in fission yeast. In budding yeast, autophagy-deficient mutants are known to be sterile, whereas in fission yeast we found that up to 30% of autophagy-defective cells with amino acid auxotrophy were able to recover sporulation when an excess of required amino acids was supplied. Furthermore, we found that approximately 15% of the autophagy-defective cells were also able to sporulate when a prototrophic strain was subjected to nitrogen starvation, which suggested that fission yeast may store sufficient intracellular nitrogen to allow partial sporulation under nitrogen-limiting conditions, although the majority of the nitrogen source is supplied by autophagy. Monitoring of the sporulation process revealed that the process was blocked non-specifically at various stages in the atg1D and atg12D mutants, possibly due to a shortage of amino acids. Taking advantage of this partial sporulation ability of fission yeast, we sought evidence for the existence of a recycling system for nitrogen sources during starvation.