Null Mutations in a Nudix Gene, ygdP, Implicate an Alarmone Response in a Novel Suppression of Hybrid Jamming

Induction of the toxic LamB-LacZ protein fusion, Hyb42-1, leads to a lethal generalized protein export defect. The prlF1 suppressor causes hyperactivation of the cytoplasmic Lon protease and relieves the inducer sensitivity of Hyb42-1. Since prlF1 does not cause a detectable change in the stability or level of the hybrid protein, we conducted a suppressor screen, seeking factors genetically downstream of lon with prlF1-like phenotypes. Two independent insertions in the ygdP open reading frame relieve the toxicity of the fusion protein and share two additional properties with prlF1: cold sensitivity and the ability to suppress the temperature sensitivity of a degP null mutation. Despite these similarities, ygdP does not appear to act in the same genetic pathway as prlF1 and Ion, suggesting a fundamental link between the phenotypes. We speculate that the common properties of the suppressors relate to secretion defects. The ygdP gene (also known as nudH) has been shown to encode a Nudix protein that acts as a dinucleotide oligophosphate (alarmone) hydrolase. Our results suggest that loss of ygdP function leads to the induction of an alarmone-mediated response that affects secretion. Using an epitope-tagged ygdP construct, we present evidence that this response is sensitive to secretion-related stress and is regulated by differential proteolysis of YgdP in a self-limiting manner.