William J. Feaver

RELATIONSHIP OF CDK-ACTIVATING KINASE AND RNA POLYMERASE II CTD KINASE TFIIH/TFIIK

KIN28, a member of the p34cdc2/CDC28 family of protein kinases, is identified as a subunit of yeast RNA polymerase transcription factor IIH (TFIIH) on the basis of sequence determination, immunological reactivity, and copurification. KIN28 is, moreover, one of three subunits of TFIIK, a subassembly of TFIIH with protein kinase activity directed toward the C-terminal repeat domain (CTD) of the largest subunit of RNA polymerase II. Itself a phosphoprotein, KIN28 interacts specifically with the two largest subunits of RNA polymerase II. Previous work of others points to two further associations: KIN28 interacts in vivo with the cyclin CCL1, and KIN28 and CCL1 are homologous to human MO15 and cyclin H, which form the cyclin-dependent kinase-activating kinase (CAK). We show that human CAK possesses the CTD kinase activity characteristic of TFIIH.

Dual roles of a multiprotein complex from S. cerevisiae in transcription and DNA repair

Yeast RNA polymerase II initiation factor b, homolog of human TFIIH, is a protein kinase capable of phosphorylating the C-terminal repeat domain of the polymerase; it possesses a DNA-dependent ATPase activity as well. The 85 kd and 50 kd subunits of factor b are now identified as RAD3 and SSL1 proteins, respectively; both are known to be involved in DNA repair. Factor b interacts specifically with another DNA repair protein, SSL2. The ATPase activity of factor b may be due entirely to that associated with a helicase function of RAD3. Factor b transcriptional activity was unaffected, however, by amino acid substitution at a conserved residue in the RAD3 nucleotide-binding domain, suggesting that the ATPase/helicase function is not required for transcription. These results identify factor b as a core repairosome, which may be responsible for the preferential repair of actively transcribed genes in eukaryotes.

Fidelity and processivity of DNA synthesis by DNA polymerase κ, the product of the human DINB1 gene

Mammalian DNA polymerase κ (pol κ), a member of the UmuC/DinB nucleotidyl transferase superfamily, has been implicated in spontaneous mutagenesis. Here we show that human pol κ copies undamaged DNA with average single-base substitution and deletion error rates of 7 × 10−3 and 2 × 10−3, respectively. These error rates are high when compared to those of most other DNA polymerases. pol κ also has unusual error specificity, producing a high proportion of T·CMP mispairs and deleting and adding non-reiterated nucleotides at extraordinary rates. Unlike other members of the UmuC/DinB family, pol κ can processively synthesize chains of 25 or more nucleotides. This moderate processivity may reflect a contribution of C-terminal residues, which include two zinc clusters. The very low fidelity and moderate processivity of pol κ is novel in comparison to any previously studied DNA polymerase, and is consistent with a role in spontaneous mutagenesis.

CTD kinase associated with yeast RNA polymerase II initiation factor b

A kinase activity specific for the C-terminal repeat domain (CTD) of RNA polymerase II is associated with nearly homogeneous yeast general initiation factor b by three criteria: cofractionation on the basis of size and charge and coinactivation by mild heat treatment. The kinase phosphorylates the CTD at multiple sites in a processive manner. Factor b may possess a DNA-dependent ATPase activity as well. Both kinase and DNA-dependent ATPase activities exhibit the same nucleotide requirements as previously demonstrated for the initiation of transcription. These results support the idea that phosphorylation of the CTD lies on the pathway of transcription initiation and identify a catalytic activity of a general factor essential for the initiation process.

Purification and Characterization of polκ, a DNA Polymerase Encoded by the Human DINB1 Gene

The Escherichia coli dinB gene encodes DNA polymerase (pol) IV, a protein involved in increasing spontaneous mutations in vivo. The protein-coding region ofDINB1, the human ortholog of DNA pol IV, was fused to glutathione S-transferase and expressed in insect cells. The purified fusion protein was shown to be a template-directed DNA polymerase that we propose to designate polκ. Human polκ lacks detectable 3′ → 5′ proofreading exonuclease activity and is not stimulated by recombinant human proliferating cell nuclear antigen in vitro. Between pH 6.5 and 8.5, human polκ possesses optimal activity at 37 °C over the pH range 6.5–7.5, and is insensitive to inhibition by aphidicolin, dideoxynucleotides, or NaCl up to 50 mm. Either Mg2+ or Mn2+ can satisfy a metal cofactor requirement for polκ activity, with Mg2+ being preferred. Human polκ is unable to bypass a cisplatin adduct in the template. However, polκ shows limited bypass of an 2-acetylaminofluorene lesion and can incorporate dCTP or dTTP across from this lesion, suggesting that the bypass is potentially mutagenic. These results are consistent with a model in which polκ acts as a specialized DNA polymerase whose possible role is to facilitate the replication of templates containing abnormal bases, or possessing structurally aberrant replication forks that inhibit normal DNA synthesis.

Human DNA Polymerase κ Bypasses and Extends beyond Thymine Glycols during Translesion Synthesis in Vitro, Preferentially Incorporating Correct Nucleotides

Human polymerase κ (polκ), the product of the human POLK (DINB1) gene, is a member of the Y superfamily of DNA polymerases that support replicative bypass of chemically modified DNA bases (Ohmori, H., Friedberg, E. C., Fuchs, R. P., Goodman, M. F., Hanaoka, F., Hinkle, D., Kunkel, T. A., Lawrence, C. W., Livneh, Z., Nohmi, T., Prakash, L., Prakash, S., Todo, T., Walker, G. C., Wang, Z., and Woodgate, R. (2001) Mol. Cell 8, 7–8; Gerlach, V. L., Aravind, L., Gotway, G., Schultz, R. A., Koonin, E. V., and Friedberg, E. C. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 11922–11927). Polκ is shown here to bypass 5,6-dihydro-5,6-dihydroxythymine (thymine glycol) generated in two different DNA substrate preparations. Polκ inserts the correct base adenine opposite thymine glycol in preference to the other three bases. Additionally, the enzyme correctly extends beyond the site of the thymine glycol lesion when presented with adenine opposite thymine glycol at the primer terminus. However, steady state kinetic analysis of nucleotides incorporated opposite thymine glycol demonstrates different misincorporation rates for guanine with each of the two DNA substrates. The two substrates differ only in the relative proportions of thymine glycol stereoisomers, suggesting that polκ distinguishes among stereoisomers and exhibits reduced discrimination between purines when incorporating a base opposite a 5R thymine glycol stereoisomer. When extending beyond the site of the lesion, the misincorporation rate of polκ for each of the three incorrect nucleotides (adenine, guanine, and thymine) is dramatically increased. Our findings suggest a role for polκ in both nonmutagenic and mutagenic bypass of oxidative damage.

The yeast TFB1 and SSL1 genes, which encode subunits of transcription factor IIH, are required for nucleotide excision repair and RNA polymerase II transcription.

The essential TFB1 and SSL1 genes of the yeast Saccharomyces cerevisiae encode two subunits of the RNA polymerase II transcription factor TFIIH (factor b). Here we show that extracts of temperature-sensitive mutants carrying mutations in both genes (tfb1-101 and ssl1-1) are defective in nucleotide excision repair (NER) and RNA polymerase II transcription but are proficient for base excision repair. RNA polymerase II-dependent transcription at the CYC1 promoter was normal at permissive temperatures but defective in extracts preincubated at a restrictive temperature. In contrast, defective NER was observed at temperatures that are permissive for growth. Additionally, both mutants manifested increased sensitivity to UV radiation at permissive temperatures. The extent of this sensitivity was not increased in a tfb1-101 strain and was only slightly increased in a ssl1-1 strain at temperatures that are semipermissive for growth. Purified factor TFIIH complemented defective NER in both tfb1-101 and ssl1-1 mutant extracts. These results define TFB1 and SSL1 as bona fide NER genes and indicate that, as is the case with the yeast Rad3 and Ss12 (Rad25) proteins, Tfb1 and Ssl1 are required for both RNA polymerase II basal transcription and NER. Our results also suggest that the repair and transcription functions of Tfb1 and Ssl1 are separable.

The RAD7, RAD16, and RAD23 genes of Saccharomyces cerevisiae: requirement for transcription-independent nucleotide excision repair in vitro and interactions between the gene products.

Nucleotide excision repair (NER) is a biochemical process required for the repair of many different types of DNA lesions. In the yeast Saccharomyces cerevisiae, the RAD7, RAD16, and RAD23 genes have been specifically implicated in NER of certain transcriptionally repressed loci and in the nontranscribed strand of transcriptionally active genes. We have used a cell-free system to study the roles of the Rad7, Rad16, and Rad23 proteins in NER. Transcription-independent NER of a plasmid substrate was defective in rad7, rad16, and rad23 mutant extracts. Complementation studies with a previously purified NER protein complex (nucleotide excision repairosome) indicate that Rad23 is a component of the repairosome, whereas Rad7 and Rad16 proteins were not found in this complex. Complementation studies with rad4, rad7, rad16, and rad23 mutant extracts suggest physical interactions among these proteins. This conclusion was confirmed by experiments using the yeast two-hybrid assay, which demonstrated the following pairwise interactions: Rad4 with Rad23, Rad4 with Rad7, and Rad7 with Rad16. Additionally, interaction between the Rad7 and Rad16 proteins was demonstrated in vitro. Our results show that Rad7, Rad16, and Rad23 are required for transcription-independent NER in vitro. This process may involve a unique protein complex which is distinct from the repairosome and which contains at least the Rad4, Rad7, and Rad16 proteins.

Genes For Tfb2, Tfb3, and Tfb4 Subunits of Yeast Transcription/Repair Factor IIH HOMOLOGY TO HUMAN CYCLIN-DEPENDENT KINASE ACTIVATING KINASE AND IIH SUBUNITS

Genes for the Tfb2, Tfb3, and Tfb4 subunits of yeast RNA polymerase transcription factor IIH (TFIIH) are described. All three genes are essential for cell viability, and antibodies against Tfb3 specifically inhibit transcription in vitro. A C-terminal deletion of Tfb2 caused a defect in nucleotide excision repair, as shown by UV sensitivity of the mutant strain and loss of nucleotide excision repair activity in cell extracts (restored by the addition of purified TFIIH). An interaction between Tfb3 and the Kin28 subunit of TFIIH was detected by the two-hybrid approach, consistent with a role for Tfb3 in linking kinase and core domains of the factor. The deduced amino acid sequence of Tfb2 is similar to that of the 52-kDa subunit of human TFIIH, while Tfb3 is identified as a RING finger protein homologous to the 36-kDa subunit of murine CAK (cyclin-dependent kinase activatingkinase) and to the 32-kDa subunit of human TFIIH. Tfb4 is homologous to p34 of human TFIIH and is identified as the weakly associated 37-kDa subunit of the yeast factor. These and other findings reveal a one-to-one correspondence and high degree of sequence similarity between the entire set of yeast and human TFIIH polypeptides.

Yeast nucleotide excision repair proteins Rad2 and Rad4 interact with RNA polymerase II basal transcription factor b (TFIIH).

The Rad2, Rad3, Rad4, and Ss12 proteins are required for nucleotide excision repair in yeast cells and are homologs of four human proteins which are involved in a group of hereditary repair-defective diseases. We have previously shown that Rad3 protein is one of the five subunits of purified RNA polymerase II basal transcription initiation factor b (TFIIH) and that Ss12 protein physically associates with factor b (W.J. Feaver, J.Q. Svejstrup, L. Bardwell, A.J. Bardwell, S. Buratowski, K.D. Gulyas, T.F. Donahue, E.C. Friedberg, and R.D. Kornberg, Cell 75:1379-1387, 1993). Here we show that the Rad2 and Rad4 proteins interact with purified factor b in vitro. Rad2 (a single-stranded DNA endonuclease) specifically interacts with the Tfb1 subunit of factor b, and we have mapped a limited region of the Rad2 polypeptide which is sufficient for this interaction. Rad2 also interacts directly with Ss12 protein (a putative DNA helicase). The binding of Rad2 and Rad4 proteins to factor b may define intermediates in the assembly of the nucleotide excision repair repairosome. Furthermore, the loading of factor b (or such intermediates) onto promoters during transcription initiation provides a mechanism for the preferential targeting of repair proteins to actively transcribing genes.