The integrity of mitosis is dependent upon strict regulation of microtubule stability and dynamics. Although much information has been accumulated on regulators of the microtubule cytoskeleton, our knowledge of the specific pathways involved is still limited. Here we designed genetic screens to identify regulators of microtubule stability that are dispensable in the wild type yet become essential under microtubule-disrupting conditions. We found that the transcriptional cofactor Swi6p and activator Swi4p, as well as the G(2)/M-specific cyclin Clb2p, are required in a microtubule-destabilizing environment. Swi6p and Swi4p can combine as a transcriptional complex, called the SBF complex (SBF for Swi4/6 cell cycle box [SCB]-binding factor) that is functionally homologous to the metazoan DP1/2-E2F complex and that controls the G(1)/S transition through the genes it regulates. We show that Swi6p's contribution to microtubule stability can be either dependent or independent of the SBF complex. The SBF-dependent pathway requires downregulation of SBF complex levels and may thereby reroute the transcriptional program in favor of greater microtubule stability. This pathway can be triggered by overexpression of Fcp1p, a phosphatase in the general transcription machinery, or by expression of an allele of SWI6 that is associated with reduced transcription from SBF-controlled promoters. The SBF-independent pathway is activated by a constitutively nuclear allele of Swi6p. Our results introduce novel roles in microtubule stability for genes whose participation in the process may be masked under normal conditions yet nonetheless acquire a dominant role when microtubule stability is compromised.
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