Robust mRNA Transcription in Chicken DT40 Cells Depleted of TAFII31 Suggests Both Functional Degeneracy and Evolutionary Divergence

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Abstract
We have employed gene targeting coupled with conditional expression to construct a chicken DT40 cell line in which a tetracycline (Tet)-repressible promoter is exclusively responsible for expression of cTAFII31, a histone-like TAFII residing in both the transcription factor TFIID and the histone acetylase complex PCAF/SAGA. Tet addition resulted in rapid loss of cTAFII31 mRNA and protein, eventually leading to apoptotic cell death. Significantly, five of six other TAFIIs tested were also rapidly depleted, but levels of the TATA binding protein and subunits of PCAF/SAGA were at most modestly compromised. Strikingly, pulse-labeling experiments indicate that total poly(A)+mRNA transcription was not significantly reduced after cTAFII31 depletion, and steady-state levels of several specific transcripts remained the same or decreased only mildly. Moreover, activation of c-fos transcription following serum starvation occurred efficiently in the absence of cTAFII31. These data, which contrast with comparable studies in yeast, strongly suggest that cTAFII31 and perhaps other TAFIIs are not essential for general mRNA transcription in DT40 cells. We propose that this is due to extensive functional degeneracy in the highly complex metazoan transcriptional machinery.