Different patterns of expression of β‐tubulin genes in Tetrahymena pyriformis during reciliation

The ciliate Tetrahymena pyriformis contains one α‐tubulin (αTT) and two β‐tubulin (βTT1 and βTT2) genes. The specific expression of these genes was investigated by Northern blot hybridization using oligonucleotide probes complementary to βTT1 and βTT2 genes and the coding region of the α‐tubulin gene. The three genes are expressed producing 1.8‐kb mRNAs but the level of βTT1 mRNA is much higher than that of βTT2 mRNA. During cilia regeneration, we found that the expression patterns of the αTT and βTT1 genes are similar whereas that of the βTT2 gene is different. The αTT and βTT1 transcripts reached higher values between 60–120 min after the onset of reciliation than in exponentially growing cells, while βTT2 transcripts were maintained at low levels during the whole period. The differences in the amounts of steady‐state populations of the both β‐tubulin mRNAs do not correspond to the copy number per haploid genome. These differences could result from the fact that the promoter region of βTT2 may contain highly structured sequences which would affect the binding of the respective trans‐acting factor(s). The apparent transcription rate revealed a significant increase at 15 min of reciliation which could be responsible for the high levels of αTT and βTT1 transcripts in the cytoplasm between 60–120 min of reciliation. This coordinated response to cilia regeneration of the αTT and βTT1 tubulin genes is also a relevant aspect of our findings. Several conserved motifs found in their promoter regions led us to think that some of them may function as cis‐elements in the specific binding of nuclear protein factor(s).