ASI2, a gene encoding a putative signal transduction receptor, is required for sexual reproduction in Tetrahymena thermophila

Ciliate protozoa have two nuclei: a germ line micronucleus and a somatic macronucleus. At sexual reproduction the parental macronucleus is degraded and a new macronucleus develops from mitotic products of the zygotic micronucleus. A gene encoding a putative signal transduction receptor, ASI2 (a&barbelow;nlagen s&barbelow;tage i&barbelow;nduced gene 2), was cloned and sequenced in Tetrahymena thermophila. ASI2 somatic knockout strains are viable, thus ASI2 is not essential for vegetative growth. The deduced Asi2p has two transmembrane domains, two coiled-coil domains, and a Cache domain similar to the sensory domain of bacterial chemotaxis receptors. The abundance of ASI2 mRNA is very low in vegetatively growing wild-type cells, but increases in starved cells and during development of the new macronucleus in mating cells. In matings between somatic ASI2 knockout cell lines, the majority of cell pairs complete meiosis and fertilization but then abort the mating. Homozygous ASI2 germ line knockout cells with wild type parental macronuclei proceed through mating but arrest at late macronuclear anlagen development stage and die before the first post-conjugation fission. DNA rearrangement occurs in matings between ASI2 germ line knockouts, but DNA replication in the macronuclear anlagen is defective. Both the somatic and the germ line null phenotypes are rescued in matings of the knockout strains with wild type cells. We propose that Asi2p is a signal transduction receptor that is required in both the somatic macronucleus and the germ line micronucleus for sexual reproduction in Tetrahymena thermophila.