Developmentally and environmentally regulated expression of gamone 1: the trigger molecule for sexual reproduction in Blepharisma japonicum

Sexual reproduction (conjugation) in protozoan ciliates is induced by specific cell-cell interactions between cells of complementary mating types. The ancestral ciliate Blepharisma japonicum has two mating types, I and II. The substances that act as signaling molecules in this extracellular interaction for conjugation are called gamones. The glycoprotein gamone 1, produced by mating type I cells, is a key factor that triggers this interaction. We have previously isolated gamone 1 and determined its complete amino acid sequence. To elucidate the mechanism of initiation of conjugation in ciliates, we investigated the transcription of the gamone 1 gene and found that it is controlled by various internal and external factors. The gamone 1 gene transcript appeared specifically when sexually mature mating type I cells were starved. It was not detected in immature cells, mating type II cells or proliferating cells. The level of transcription was markedly increased in type I cells when they were stimulated with gamone 2, which is secreted by type II cells. This is the first report that the transcription of gamone genes in ciliates is strictly regulated by developmental and environmental factors. This study suggests that the onset of transcription of gamone 1 is linked to the switching mechanism that converts mitotically proliferating cells to differentiated preconjugants, the mechanism of differentiation from immature to mature cells in clonal development, and the mechanism that ensures mating type-specific gene silencing.