A gene, zyg1, was isolated by differential screening from Dictyostelium mucoroides-7 (Dm7) cells, as one preferentially expressed during their sexual development. The zyg1 gene encodes for a novel protein (ZYG1; deduced Mr 29.4 x 10(3)) consisting of 268 amino acids. Although the ZYG1 protein has predicted PKC phosphorylation sites, it has neither transmembrane domains nor specified signal sequences. The expression of zyg1 was initiated after 2 h of starvation and reached its maximum level at 8 h under submerged conditions. The expression pattern is quite similar to the temporal change of zygote formation during sexual development (macrocyst formation) with 1 h of precedence. The zyg1 mRNA in Dm7 cells developing on agar was retained until zygotes were formed. Zyg1-overexpressing cells derived from Dm7 cells eventually formed many loose mounds, in which giant cells were surrounded by normal-sized cells, in addition to mature macrocysts even under the conditions favouring for asexual sorocarp formation. Giant cells were found by DAPI-staining to be multinucleate, possibly because of the precocious overexpression of zyg1 mRNA. Western blottings using a specific antibody raised against the oligopeptide near the C-terminal region of ZYG1 also showed that ZYG1 was actually over-produced in the zyg1-overexpressing cells. From these results, it is evident that the zyg1 gene has an essential role in zygote formation by inducing sexual cell fusion.