Tetrahymena actin. Cloning and sequencing of the Tetrahymena actin gene and identification of its gene product.

Actin is ubiquitous in eukaryotes, nevertheless its existence has not yet been clearly proven in Tetrahymena. Here we report the cloning and sequencing of an actin gene from the genomic library of Tetrahymena pyriformis using a Dictyostelium actin gene as a probe. The Tetrahymena actin gene has no intron. The predicted actin is composed of 375 amino acids like other actins and its molecular weight is estimated as 41,906. Both T. pyriformis and T. thermophila possess a single species of actin genes which differ in their restriction patterns. Northern hybridization analysis revealed that the actin gene was actively transcribed in vivo. To detect the gene product, we synthesized an N-terminal peptide of the deduced sequence and prepared its antibody. Using an immunoblotting technique, we identified Tetrahymena actin on a two-dimensional gel electrophoretic plate. The actin spot migrated near an added spot of rabbit skeletal muscle actin, but clearly differed from the latter in its isoelectric point and apparent molecular weight. The primary structure of Tetrahymena actin shares about 75% homology equally with those of other representative actins. This value is extremely low as a homology rate between known actins. Tetrahymena actin diverges not only in relatively variable regions of other actins, but also in relatively constant regions. The hydrophilicity levels of two regions (residues 190 to 200 and residues 225 to 235) are also quite different between the Tetrahymena actin and skeletal muscle actin. Thus, we conclude that actin is present in Tetrahymena, but it is one of the most unique actins among the actins known hereto.