Nucleotide sequence and expression of two β-tubulin genes in Stylonychia lemnae

Abstract

The gene-sized macronuclear DNA of the hypotrichous ciliate Stylonychia lemnae contains one size class of DNA molecules of 1.85kb (1 kb = 10 3 base-pairs) coding for β-tubulin. These DNA molecules consist of two different β-tubulin genes, β 1 and β 2, which are amplified to about 150,000 ( β 1) and 30,000 ( β 2) copies per macronucleus. Both genes were cloned and sequenced entirely. The coding sequences of the two molecules (1329 base-pairs including TGA) predict identical amino acid sequences for the proteins and show a nucleotide homology of 97.2%. The nucleotide as well as the encoded amino acid sequences are highly conserved, when compared to β-tubulin genes from vertebrates. The ciliatespecific codon TAA specifying glutamine is present only in the β 2-tubulin gene, whereas glutamine is encoded soley by CAA in the β 1-tubulin gene. The 5′ and 3′-non-coding regions of both β-tubulin genes are similar in length, but differ extremely in nucleotide sequence. Both β-tubulin genes are transcriptionally active in S. lemnae, although not all putative transcription-regulatory sequences known from higher eukaryotes can be detected within the non-coding regions. The two transcription products localized by S 1-mapping experiments show a similar length of about 1.40 kb and transcription seems to be regulated differently for β 1 and β 2.