A macronuclear gene-sized molecule carrying an actin gene from the hypotrich ciliate, Histriculus cavicola, was characterized. Southern blot analysis using a coding region probe suggested that actin in H. cavicola is encoded by a single gene. A comparison of the promoter regions indicated that the H. cavicola actin gene has a TATA box in the 5' flanking region in a position identical to those in other oxytrich ciliates. The coding sequence of this gene is not interrupted by any introns, and codes for a protein of 375 amino acid residues. This protein shares a high degree of similarity with other oxytrichid actins, and a relatively low similarity with actins from other eukaryotes. Comparative analyses of sequences indicated that most of the amino acid substitutions in hypotrich actins are found in surface loops, while the core structures are well-conserved. The sites that interact with DNase I and several regions involved in actin-actin contact have diverged considerably in hypotrich actins, while nucleotide-binding sites are the best-conserved interaction motif.
The polymerase chain reaction was used to amplify and clone an actin gene fragment from Paramecium tetraurelia. This DNA fragment was 1,138 bp long, more than 96% of the actin coding sequence, and contained four in-frame UAA codons and two small introns located at positions unique in the actin intron catalogue. This is the first report for the phylum Ciliophora of an actin gene containing introns. The deduced amino acid sequence of this actin fragment shared 58-77% identity with other actins. When compared with rabbit alpha-muscle actin, similarities were observed mainly in subdomains 1 and 3, whereas subdomains 2 and 4 appeared to be more divergent.
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