ABSTRACT
A single CSB dimer seems to wrap approximately 125 bp D N A in a left handed manner and the wrapping and unwrapping is affected by ATP binding.2In addition to the ATPase activity, CSB has been shown to exhibit both strand exchange and single strand annealing activity, which, at least for the latter, is independent o f ATPase hydrolysis and is inhibited by binding o f an un-hydrolysable ATP analogue.11 Strand annealing has otherwise been associated with Rad51 in homologous recombination where it stimulates invasion o f the presynaptic filament into homologous dsDNA in a manner which is assisted by its paralogs Rad52 and Rad54.12 The strand annealing activity o f CSB is inhibited by the single strand binding protein RPA,11 analogous to the inhibitory effect o f RPA on W RN (Werner syndrome), BLM (Bloom syndrome) and RecQ5|3 strand annealing activities.13,14 The biological role o f the strand annealing and strand exchange activities o f CSB is still unknown.An in vitro study has reported ATPase independent topological changes o f a plasmid as a consequence o f CSB binding.3 Another study finds that CSB introduces negative supercoils in a plasmid and shortens the contour length o f the plasmid with approximately 125 bp. Both functions were dependent on ATP binding but no t hydrolysis.2 Together, the shortening and the topological change suggest that CSB wraps D N A around it in a manner very similar to a nucleosome.2 Data from D N A binding and ATPase assays indicate that binding and hydrolysis o f ATP by CSB results in D N A binding and release.2,10 It is, however, no t clear whether D N A binding is triggered upon ATP binding or hydrolysis, but a mechanism with alternating D N A binding and release in a manner, which is regulated by ATP hydrolysis, would open the possibility o f CSB to track along D N A and alter its topology by wrapping o f the D N A. In line w ith this, CSB has been reported to mediate ATPase dependent nucleosome remodeling o f a mononucleosome, which did no t result in core histone release.3 Nevertheless, release o f histones is no t a prerequisite for chromatin remodeling to have an effect on D N A repair since chromatin remodelers, that do not release histones but only opens up the chromatin structure, have been demonstrated to stimulate base excision repair (BER).15 CSB and TranscriptionIn vivo transcription in CSB deficient cells have been reported to be reduced by 50% compared to wild type cells and in vitro transcription studies on chromatin from CSB deficient cells showed that transcription elongation was significantly decreased in the absence o f CSB. Furthermore, when compared to CSB deficient cells, the transcription from chromatin in wild type cells was more resistant to the detergent Triton X-100, indicating that the transcription machinery is less tightly associated w ith the chromatin in CSB deficient cells compared to wild type cells.16 Weiner et al reported that the transcription pattern o f CSB deficient cells overlaps significantly w ith the transcription pattern o f cells treated with the histone deacetylase (H D A C ) inhibitor TSA, indicating that CSB could play a role in chromatin maintenance.17 The similarity between H D A C inhibited cells and CSB deficient cells thus indicates that the role o f CSB is to decrease the am ount o f histone acetylation. In contrast, the acetylation o f histone H 4 is decreased near certain prom oters in CSB deficient cells before and after U V irradiation, suggesting the existence o f CSB dependent histone acetyl transferase (HAT) activity as argued by Egly et al.5 Thus, the available evidence indicates that the CSB status o f a cell affects the chromatin structure, but it remains unclear whether CSB is involved in increasing or decreasing the histone acetylation. The apparent contradiction could, however, be explained by the methods employed in the two studies. Weiner et al investigated the transcription pattern by microarray analysis, whereas Egly et al investigated the acetylation status o f the prom oter region o f a few specific genes. Therefore, the difference between the general pattern throughout the genome and the pattern for specific genes could render a possible explanation for the diverging results. This notion on regulation o f specific genes is supported by a microarray study investigating the transcriptional response to oxidative stress in CSB deficient cells compared to CSB complemented cells.18 The observation here was that CSB is needed for the specific regulation o f the transcription pattern after oxidative stress,18 supporting that different genes are regulated differendy by CSB.
