DNA damage-induced mutation: tolerance via translesion synthesis.

Translesion synthesis (TLS) appears to be required for most damage-induced mutagenesis in the yeast Saccharomyces cerevisiae, whether the damage arises from endogenous or exogenous sources. Thus, the production of such mutations seems to occur primarily as a consequence of the tolerance of DNA lesions rather than an error-prone repair mechanism. Tolerance via TLS in yeast involves proteins encoded by members of the RAD6 epistasis group for the repair of ultraviolet (UV) photoproducts, in particular two non-essential DNA polymerases that catalyse error-free or error-prone TLS. Homologues of these RAD6 group proteins have recently been discovered in rodent and/or human cells. Furthermore, the operation of error-free TLS in humans has been linked to a reduced risk of UV-induced skin cancer, whereas mutations generated by error-prone TLS may increase the risk of cancer. In this article, we review and link the evidence for translesion synthesis in yeast, and the involvement of nonreplicative DNA polymerases, to recent findings in mammalian cells.

Evidence from mutational specificity studies that yeast DNA polymerases delta and epsilon replicate different DNA strands at an intracellular replication fork.

Although polymerases delta and epsilon are required for DNA replication in eukaryotic cells, whether each polymerase functions on a separate template strand remains an open question. To begin examining the relative intracellular roles of the two polymerases, we used a plasmid-borne yeast tRNA gene and yeast strains that are mutators due to the elimination of proofreading by DNA polymerases delta or epsilon. Inversion of the tRNA gene to change the sequence of the leading and lagging strand templates altered the specificities of both mutator polymerases, but in opposite directions. That is, the specificity of the polymerase delta mutator with the tRNA gene in one orientation bore similarities to the specificity of the polymerase epsilon mutator with the tRNA gene in the other orientation, and vice versa. We also obtained results consistent with gene orientation having a minor influence on mismatch correction of replication errors occurring in a wild-type strain. However, the data suggest that neither this effect nor differential replication fidelity was responsible for the mutational specificity changes observed in the proofreading-deficient mutants upon gene inversion. Collectively, the data argue that polymerases delta and epsilon each encounter a different template sequence upon inversion of the tRNA gene, and so replicate opposite strands at the plasmid DNA replication fork.

The assembly of yeast mitochondrial ATP synthase: subunit depletion in vivo suggests ordered assembly of the stalk subunits b, OSCP and d.

The abundance in vivo of each of three subunits b, OSCP and d, components of the stalk region of the yeast mitochondrial ATP synthase complex, was manipulated by a controlled depletion strategy. Western blots of whole cell lysates were used to study the effect of depletion of each of these subunits on the cellular levels of other subunits of the enzyme complex. A hierarchy of subunit stability was determined and interpreted to indicate the order of assembly of these three subunits of the stalk region. Thus, subunit b is assembled first, followed by OSCP and then by subunit d.

Rescue of yeast defective in mitochondrial ATP synthase subunit 8 by a heterologous gene from Aspergillus nidulans.

Mitochondrial ATP synthase subunit 8 of the yeast Saccharomyces cerevisiae and of the filamentous fungus Aspergillus nidulans have the same length and similar structural motifs. However, the two proteins share only 50% identical residues, with the conserved residues being concentrated in the N- and C-terminal domains. We have investigated whether it is amino acid sequence or overall structural motifs that are required for subunit 8 function. PCR was used to construct a gene encoding A. nidulans subunit 8 fused to an N-terminal cleavable mitochondrial targeting sequence. Following expression in the nucleus of a yeast strain deficient in subunit 8, the chimaeric precursor targeted the subunit 8 protein back to the mitochondrion. The A. nidulans subunit 8 was found to be able to restore growth on non-fermentable substrate at 18 degrees C and 28 degrees C, but not at 36 degrees C. Given the sequence divergence between subunit 8 of A. nidulans and that of S. cerevisiae, this finding suggests that common structural motifs are important for subunit 8 function.