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Gut granules are cell type-specific lysosome-related organelles found within the intestinal cells of Caenorhabditis elegans. To investigate the regulation of lysosome-related organelle size, we screened for C. elegans mutants with substantially enlarged gut granules, identifying alleles of the vacuolar-type H+-ATPase and uridine-5′-monophosphate synthase (UMPS)-1. UMPS-1 catalyzes the conversion of orotic acid to UMP; this comprises the two terminal steps in de novo pyrimidine biosynthesis. Mutations in the orthologous human gene UMPS result in the rare genetic disease orotic aciduria. The umps-1(−) mutation promoted the enlargement of gut granules to 250 times their normal size, whereas other endolysosomal organelles were not similarly affected. UMPS-1::green fluorescent protein was expressed in embryonic and adult intestinal cells, where it was cytoplasmically localized and not obviously associated with gut granules. Whereas the umps-1(−) mutant is viable, combination of umps-1(−) with mutations disrupting gut granule biogenesis resulted in synthetic lethality. The effects of mutations in pyr-1, which encodes the enzyme catalyzing the first three steps of de novo pyrimidine biosynthesis, did not phenotypically resemble those of umps-1(−); instead, the synthetic lethality and enlargement of gut granules exhibited by the umps-1(−) mutant was suppressed by pyr-1(−). In a search for factors that mediate the enlargement of gut granules in the umps-1(−) mutant, we identified WHT-2, an ABCG transporter previously implicated in gut granule function. Our data suggest that umps-1(−) leads to enlargement of gut granules through a build-up of orotic acid. WHT-2 possibly facilitates the increase in gut granule size of the umps-1(−) mutant by transporting orotic acid into the gut granule and promoting osmotically induced swelling of the compartment.