Characterization of two types of histone H2B genes from macronuclei of Tetrahymena thermophila.

Two histone H2B gene clones were isolated from macronuclei of Tetrahymena thermophila. Nucleotide sequences of the two clones were highly homologous within the coding region but not in the noncoding region. Comparison of the deduced amino acid sequences between the two clones showed three differences in a total of 121 amino acids. Each of the two clones contained a TAA triplet within the coding region, which appeared to code for a glutamine residue. To demonstrate the existence of histone mRNA containing UAA triplet, nuclease P1 protection mapping using total cellular RNA and nucleotide sequencing of primer extension products were carried out. The results clearly indicated that two cloned histone H2B genes were transcribed, giving rise to the major histone H2B mRNAs with a UAA triplet sequence in frame. The tentative 5'- and 3'-ends of histone H2B mRNAs were determined.