The nematode Caenorhabditis elegans uses simple building blocks from primary metabolism and as trategy of modular assembly to build ag reat diversity of signaling molecules,t he ascarosides,w hichf unction as ac hemical language in this model organism. In the ascarosides,t he dideoxysugar ascarylose serves as as caffold to which diverse moieties from lipid, amino acid, neurotransmitter,and nucleoside metabolism are attached. However,t he mechanisms that underlie the highly specific assembly of ascarosides are not understood. We show that the acyl-CoA synthetase ACS-7, which localizes to lysosome-related organelles,i ss pecifically required for the attachment of different building blocks to the 4'-position of ascr#9. We further showt hat mutants lacking lysosome-related organelles are defective in the production of all 4'-modified ascarosides,thus identifying the waste disposal system of the cell as ahotspot for ascaroside biosynthesis.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: http://dx.