Is apparent autoregulatory control of tubulin synthesis nontranscriptionally regulated?

Virtually all higher eucaryotic cells rapidly depress synthesis of new alpha- and beta-tubulin polypeptides in response to microtubule inhibitors that increase the pool of depolymerized subunits. This apparently autoregulatory control of tubulin synthesis is achieved through modulation of tubulin messenger RNA levels. In particular, in cells treated with the microtubule-depolymerizing drug colchicine, tubulin messenger RNAs are specifically and rapidly lost from the cell cytoplasm. A priori this loss may be the result of suppression of new tubulin RNA transcription, failure of newly synthesized tubulin RNAs to be properly processed or transported from the nucleus, or an increased rate of cytoplasmic tubulin RNA degradation. Although transcriptional regulation has been demonstrated for most cellular eucaryotic genes thus far investigated in detail, we found that the apparent rates of tubulin RNA transcription were essentially unchanged in isolated nuclei derived from colchicine treated or control cells. This finding argues that the principal control of tubulin gene expression in response to altered subunit pools is probably not achieved through a transcriptionally regulated mechanism.