The ?-tubulin gene family of pea: Primary structures, genomic organization and intron-dependent evolution of genes

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Abstract
One gene and two cDNAs encoding three different β-tubulins (TUB1, TUB2, TUB3) of pea have been cloned and sequenced. The derived amino acid sequences show between 92% and 96% identity relative to one another and to most other β-tubulins of higher plants and green algae. Two notable extremes are the high similarity of 98% between pea TUB3 and maize β-tubulin 2 and the relatively low similarity (90%) of the hypocotyl-specific β-tubulin 1 of soybean to the pea sequences. These similarities do not reflect the molecular phylogeny but rather differences in evolutionary rate of β-tubulins which are differentially regulated during plant development. Genomic Southern blots reveal a β-tubulin gene family in pea with at least four separate members including two TUB1 genes, one TUB2 gene and one TUB3 gene. This contradicts an earlier report by Rahaet al. (Plant Mol Biol 9: 565–571, 1987) suggesting a tandem repeat organization of tubulin genes in pea. The pea TUB1 gene has two introns in identical positions compared to the β-tubulin genes fromArabidopsis and soybean. In an attempt to reconstruct the universal ancestor of all present-day tubulin genes the intron positions in 38 different α- and β-tubulin genes from plants, animals, fungi and protozoa were compared. This comparison shows that the primordial gene probably had many introns (more than 20) separating ‘protoexons’ of 15 to 20 codons in agreement with the ‘exon theory of genes’. It also supports the view that, during the course of evolutions introns have shifted and were deleted preferentially in the 3′ part of the genes. Similar observations have been made previously for other genes. They can be interpreted in terms of a homologous recombination of genes with their modified (incorrectly spliced) and reverse-transcribed pre-mRNAs.