Involvement of yeast carboxy-terminal domain kinase I (CTDK-I) in transcription elongation in vivo.

Yeast cells lacking transcription elongation factor genes such as PPR2 (TFIIS) and ELP (Elongator) are viable and show deleterious phenotypes only when transcription is rendered less effective by RNA polymerase mutations or by decreasing nucleotide pools. Here we demonstrate that deletion of the CTK1 gene, encoding the kinase subunit of RNA polymerase II carboxy-terminal domain kinase I (CTDK-I), is synthetically lethal when combined with deletion of PPR2 or ELP genes. The inviability of ctk1 elp3 double mutants can be rescued by expression of an Elp3 mutant that has retained its ability to form the Elongator complex but has severely diminished histone acetyltransferase activity, suggesting that the functional overlap between CTDK-I and Elongator is in assembly of RNA polymerase II elongation complexes. Our results suggest that CTDK-I plays an important role in transcriptional elongation in vivo, possibly by creating a form of RNA polymerase that is less prone to transcriptional arrest.

Overlapping roles for the histone acetyltransferase activities of SAGA and elongator in vivo.

Elp3 and Gcn5 are histone acetyltransferases (HATs) that function in transcription as subunits of Elongator and SAGA/ADA, respectively. Here we show that mutations that impair the in vitro HAT activity of Elp3 confer typical elp phenotypes such as temperature sensitivity. Combining an elp3Delta mutation with histone H3 or H4 tail mutations confers lethality or sickness, supporting a role for Elongator in chromatin remodelling in vivo. gcn5Deltaelp3Delta double mutants display a number of severe phenotypes, and similar phenotypes result from combining the elp mutation with mutation in a gene encoding a SAGA-specific, but not an ADA-specific subunit, indicating that Elongator functionally overlaps with SAGA. Because concomitant active site alterations in Elp3 and Gcn5 are sufficient to confer severe phenotypes, the redundancy must be specifically related to the HAT activity of these complexes. In support of this conclusion, gcn5Deltaelp3Delta phenotypes are suppressed by concomitant mutation of the HDA1 and HOS2 histone deacetylases. Our results demonstrate functional redundancy among transcription-associated HAT and deacetylase activities, and indicate the importance of a fine-tuned acetylation-deacetylation balance during transcription in vivo.

A novel histone acetyltransferase is an integral subunit of elongating RNA polymerase II holoenzyme.

The elongator complex is a major component of the RNA polymerase II (RNAPII) holoenzyme responsible for transcriptional elongation in yeast. Here we identify Elp3, the 60-kilodalton subunit of elongator/RNAPII holoenzyme, as a highly conserved histone acetyltransferase (HAT) capable of acetylating core histones in vitro. In vivo, ELP3 gene deletion confers typical elp phenotypes such as slow growth adaptation, slow gene activation, and temperature sensitivity. These results suggest a role for a novel, tightly RNAPII-associated HAT in transcription of DNA packaged in chromatin.