We study Siw14 in fission yeast. Siw14 is an inositol pyrophosphatase that also has polyphosphatase activity.  Knockout of Siw14 and Aps1 is synthetically lethal. We demonstrate that this is due to the generation of high levels of InsP8. Overzealous transcription termination is the basis for 1,5-IP8 toxicosis.