α-Tubulin gene family of maize (Zea mays L.). Evidence for two ancient α-tubulin genes in plants

Among 81 α-tubulin cDNA clones prepared from RNA from maize seedling shoot, endosperm and pollen, we identified six different α-tubulin coding sequences. The DNA sequence analysis of coding and non-coding regions from the clones showed that they can be classified into three different α-tubulin gene subfamilies. Genes within each subfamily encode proteins that are 99 to 100% identical in amino acid sequence. Deduced amino acid sequence identity between genes in different subfamilies ranges from 89 to 93 %. The results of hybridizations of genomic DNAs to α-tubulin coding region probes and to 3′ non-coding region probes constructed from six different α-tubulin cDNA clones indicated that the maize α-tubulin gene family contains at least eight members. Comparison of deduced α-tubulin amino acid sequences from maize and the dicot species Arabidopsis thaliana showed that α-tubulin isotypes encoded by genes in maize subfamilies I and II are more similar to specific Arabidopsis gene products (96 to 97% amino acid identity) than to isotypes encoded by genes in the other maize subfamilies. Phylogenetic analyses revealed that genes in these two subfamilies were derived from two ancient α-tubulin genes that predate the divergence of monocots and dicots. These same analyses revealed that the gene in maize subfamily III is more closely related to sequences from subfamily I genes than to those from subfamily IT genes. However, the subfamily III gene has no close counterpart in Arabidopsis. We found evidence of a transposable element-like insertion in the subfamily III gene in some maize lines.