A gene from Phytophthora infestans that was previously identified as being induced during the development of sexual spores was also found to be active during asexual sporulation. The gene, M90, was expressed as a 3.1-kb primary transcript containing two introns and was predicted to encode a member of the Puf family of translational regulators. The protein showed up to 51% amino acid identity to other Puf proteins within its 353-amino-acid RNA-binding domain. Little similarity extended beyond this region, as noted for other members of the family. Expression of M90 was measured by using RNA blots and transformants of P. infestans expressing a fusion between the M90 promoter and the -glucuronidase (GUS) gene. A 1.3-kb promoter fragment conferred the normal M90 pattern of expression to the GUS reporter in transformants. In matings, expression was first detected in male and female gametangial initials and persisted in mature oospores. Expression was also observed in hyphal tips just prior to asexual sporulation, in sporangiophores, in mature sporangia, and in zoospores. The signal quickly disappeared once spores made the transition to hyphae after germination. Nutrient limitation did not induce the gene. Potential roles for a translational regulator during both sexual development and asexual sporulation are discussed.Mating, sexual sporulation, and asexual sporulation are central to the life histories of many lower eukaryotes. Mating is important because this increases genetic fitness and diversity and because sexual spores are often thick walled and thus more durable than vegetative structures. Also, both sexual and asexual spores are the main agents of dispersal for many species. Detailed molecular analyses of these pathways have been executed only for selected groups, particularly the cellular slime molds, true fungi such as ascomycetes and basidiomycetes, and green algae (2,5,10,13,31). Very limited information is available for most other lower eukaryotes, including oomycetes.Most oomycetes, which include important parasites of plants and animals, undergo both sexual and asexual sporulation. Despite their generally filamentous, fungus-like pattern of growth, oomycetes are diploid and share a close taxonomic relationship with chrysophytes, diatoms, and brown algae rather than with true fungi (3). One of the best-known oomycetes is Phytophthora infestans, which caused the Irish potato famine in the 1840s. The late blight diseases caused by P. infestans continue to have devastating effects on agricultural production and threaten food security (12).The physiology of sporulation in oomycetes is well described, but little is known at a molecular level. In heterothallic Phytophthora, sexual development occurs in response to hormonal interactions between A1 and A2 mating types (4,16,19). Male and female gametangia termed antheridia and oogonia form, and then oogonia penetrate antheridia and develop into oospores. Meiosis occurs in both gametangia, after which haploid antheridial and oogonial nuclei fuse to form a zygote...