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DNA repair ͉ MAP kinase ͉ Csb ͉ transcription-coupled repair ͉ tumorigenesis C ells use numerous mechanisms to neutralize various reactive species before they can damage DNA, as well as overlapping pathways to repair the damage, thus protecting genomic integrity. DNA damage can result in a number of potential outcomes for a mammalian cell (1), including permanent fixation of the damage during replication, leading to a heritable mutation. Replication-centric models of mutagenesis have provided valuable information regarding routes of mutagenesis under conditions of continuous cell growth and replication. These models, however, largely ignore transcription, the other major nucleic acid transaction essential to the cell. In slowly growing or nondividing cells, in which DNA replication is greatly diminished or altogether absent (e.g., terminally differentiated mammalian cells [2]), transcription must continue to provide the cell with the proteins necessary for normal physiologic processes. As such, the interaction of DNA damage with the transcription machinery occurs more frequently than with the replication apparatus.Many types of DNA damage are repaired with greater efficiency if they occur in the template strand of a transcribed gene rather than the nontemplate strand or a nontranscribed region of the genome (3, 4). Bulky lesions that distort the DNA helix will arrest RNA polymerase (RNAP) at the site of damage, promoting transcription-coupled repair (TCR) and allowing the generation of full-length, normal transcripts (5, 6). However, frequently occurring nonbulky lesions, such as 8-oxoguanine

In higher eukaryotes, DNA polymerase (pol) ␤ resides in the nucleus and participates primarily in DNA repair. The DNA polymerase ␤ from the trypanosomatid Crithidia fasciculata, however, was the first mitochondrial enzyme of this type described. Upon searching the nearly completed genome data base of the related parasite Trypanosoma brucei, we discovered genes for two pol ␤-like proteins. One is ϳ70% identical to the C. fasciculata pol ␤ and is likely the homolog of this enzyme. The other, although ϳ30% identical within the polymerase region, has unusual structural features including a short C-terminal tail and a long N-terminal extension rich in prolines, alanines, and lysines. Both proteins, when expressed recombinantly, are active as DNA polymerases and deoxyribose phosphate lyases, but their polymerase activity optima differ with respect to pH and KCl and MgCl 2 concentrations. Remarkably, green fluorescent protein fusion proteins and immunofluorescence demonstrate that both are mitochondrial, but their locations with respect to the mitochondrial DNA (kinetoplast DNA network) in this organism are strikingly different.

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