Helicases and DNA dependent ATPases of Sulfolobus solfataricus

Abstract

DNA is susceptible to various types of damage as a result of normal cellular metabolism or from environmental sources. In order to maintain genome stability a number of different, partially overlapping DNA repair pathways have evolved to tackle specific lesions or distortions in the DNA. Nucleotide excision repair (NER) is highly conserved throughout eukarya, bacteria and archaea and predominantly targets lesions that result from exposure to UV light, for example cyclobutane pyrimidine dimers and 6-4 photoproducts. The majority of archaea possess homologous of the eukaryotic repair genes and this thesis describes the isolation and the characterization of two XPB homologues identified in the crenarchaeon Sulfolobus solfataricus, SsoXPB1 and SsoXPB2. Human XPB is one of 10 proteins that make up the TFIIH transcription complex. The activity of XPB is tightly controlled by protein interactions, in particular with p52, which stimulates the ATPase activity of XPB. Rather than a conventional helicase, human XPB is thought to act as an ATP dependent conformational switch. Consistent with human XPB, however, the S. solfataricus proteins were unable to catalyse strand separation and the identification of an archaeal protein partner, Bax1, for SsoXPB2 was one of the focuses of this project. In order to maintain genome stability, the DNA must be replicated accurately with each cell cycle. When the advancing replication fork stalls at a lesion or a DNA break, it is crucial that the fork is reset and that replication continues to completion. The helicase Hel308 is thought to clear the lagging strand template of a stalled replication fork in order for replication restart to proceed via homologous recombination (HR). Although the specific function of Hel308 is not well understood, the possibilities are described in this thesis. Strand exchange proceeds to form a D-loop, followed by branch migration to increase regions of heterology during the synapsis stage of HR. No motors for branch migration have previously been recognised in archaea, although the identification of a possible candidate was investigated during this project.