Some bacteria uniquely produce "bacterial polyynes", which possess a conjugated C≡C bond starting with a terminal alkyne, and use them as chemical weapons against hosts and competitors. Cab57, a biocontrol agent against plant pathogens, has an orphan biosynthetic gene cluster for bacterial polyynes (named protegenins). In this study, the isolation, structure elucidation, and biological characterization of protegenins A-D are reported. The structures of protegenins A-D determined by spectroscopic and chemical techniques were octadecanoic acid derivatives possessing an ene-tetrayne, ene-triyne-ene, or ene-triyne moiety. The protegenins exhibited weak to strong antioomycete activity against OPU774. The deletion of , a protegenin biosynthetic gene, resulted in the reduction of the antioomycete activity of . The Gac/Rsm system, a quorum sensing-like system of bacteria, regulated the production of protegenins. The production profile of protegenins was dependent on the culturing conditions, suggesting a control mechanism for protegenin production selectivity. suppressed the damping-off of cucumber seedlings caused by , and this protective effect was reduced in the -deletion mutant. Altogether, protegenins are a new class of bacterial polyynes which contribute to the antioomycete and plant-protective effects of .