Robust mRNA transcription in chicken DT40 cells depleted of TAF(II)31 suggests both functional degeneracy and evolutionary divergence.

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 cTAF(II)31, a histone-like TAF(II) residing in both the transcription factor TFIID and the histone acetylase complex PCAF/SAGA. Tet addition resulted in rapid loss of cTAF(II)31 mRNA and protein, eventually leading to apoptotic cell death. Significantly, five of six other TAF(II)s 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 cTAF(II)31 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 cTAF(II)31. These data, which contrast with comparable studies in yeast, strongly suggest that cTAF(II)31 and perhaps other TAF(II)s 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.