Genetic characterization of the secretory mutant MS-1 of Tetrahymena thermophila: vacuolarization and block in secretion of lysosomal hydrolases are caused by a single gene mutation.

The genetics and phenotypic features (light and electron microscopy) of a secretory mutant, MS-1 of Tetrahymena thermophila blocked in secretion of lysosomal acid hydrolases have been analyzed. Although blocked constitutively in secretion, MS-1 contains active lysosomal hydrolases in amounts equivalent to the wild type. The 3:1 segregation in F-2 in sib crosses and the 1:1 segregation in test crosses indicate that the block in secretion of lysosomal hydrolases is controlled by a recessive single gene locus termed sec. The sec allele of MS-1 proved also to be responsible for the highly vacuolarized phenotype the mutant developed when it was transferred from nutrient medium into buffers of low ionic strength. Deletion mapping by crossing MS-1 with nullisomic strains, all secreting lysosomal hydrolases at wild-type rates, was performed. The sec phenotype was expressed in monosomic-4 progeny only, indicating that the sec allele is located on chromosome 4 of T. thermophila.