Chlamydomonas reinhardtii tubulin gene structure.
DNA sequencing studies have provided a picture of the total information available at the gene level for tubulin production in C. reinhardtii. The data indicates that diversity at the gene level is very limited and that all the microtubules in the cell are composed of a very similar set of tubulins. These studies contrast with similar studies of S. pombe alpha-tubulin genes and chicken beta-tubulin genes that show much heterogeneity among members of the same gene family. Further studies will be needed to investigate whether the high degree of conservation of tubulin genes is unique or common among lower eukaryotes, and what mechanisms are used to maintain homogeneity in C. reinhardtii tubulin gene families. Our DNA sequence analysis, in addition to the work of Brunke et al., has provided information on the noncoding, and possibly regulatory, portions of the tubulin genes. For example, the promoter regions of the 4 tubulin genes share a consensus sequence of 16 nucleotides upstream of the TATA box. This sequence could be involved in regulating the coordinate expression of the genes. Although little homology exists generally in the noncoding region of the genes, striking homology between the third IVS in each beta-tubulin gene is observed. Small elements homologous to the beta-tubulin IVS 3 also exist in the second IVS of each alpha-tubulin gene. In addition, considerable homology in the 5' noncoding portion of the alpha-tubulin transcripts has been noted. These homologies may be the result of recent gene conversion events, and may not have functional significance. The possibility, however, must also be considered in future experiments that these elements may play a role in regulating the expression of the tubulin genes.