Nucleotide sequence of a macronuclear DNA molecule coding for α-tubulin from the ciliate Stylonychia lemnae. Special codon usage: TAA is not a translation termination codon

Abstract

The gene-sized macronuclear DNA of the hypotrichous ciliate Stylonychia lemnae contains two size classes of DNA molecules (1.85 and 1.73 kbp) coding for alpha-tubulin. Each macronucleus contains about 55000 copies of the 1.85 kbp molecules and about 17000 copies of the 1.73 kbp DNA molecules. Five macronuclear molecules of these sequences were cloned and sequenced, one, from the 1.85 kbp size class in its entirety. The 5 sequences fell into two classes suggesting that Stylonychia lemnae contains at least two different alpha-tubulin genes. All 5 clones show the codon TAA in the same nucleotide positions of the coding region. In this position the TAA codon cannot function as a translational stop codon and we suggest that this codon codes for the amino acid glutamine. The nucleotide sequence of the coding region as well as the encoded amino acid sequence is highly conserved compared to alpha-tubulin genes from vertebrates. The noncoding regions show several putative transcription-regulatory sequences as well as sequences presumably functioning as replication origins.