Unlike pyrimidine dimers or (6–4) photoproducts, DSBs directly result in cell death; therefore, we cannot exclude the possibility that the increase in the UV sensi- tivity of recJ suggests the involvement of RecJ in DSB repair. DSB repair, in bacteria, is performed mainly by homologous recombination. In general, there are two pathways in bacterial homologous recombination: the RecBCD and RecF pathways (67,68). The RecBCD complex and another 50–30 ssExo process the termini of dsDNAs to generate the 30-overhanging structure in the RecBCD and RecF pathways, respectively. As T. thermophilus HB8 lacks the genes encoding RecBCD, the RecF pathway is expected to be dominant in this bac- terium, and 50–30 exonuclease activity of RecJ may be required for the end-resection step in this pathway (Figure 8C). In conclusion, the results of our in vivo and in vitro ex- periments suggest that 30–50 ssExo Tth ExoI participates in the excision step of MMR and the repair of deaminated bases, while 50–30 ssExo RecJ is involved in the excision step of MMR and the repair of UV-C-induced damages. The need for Tth ExoI and RecJ in other repair pathways such as the repair of methylated bases remains to be investigated. It is also to be examined whether Tth ExoI plays other roles in addition to DNA repair. Interestingly, it was recently reported that phage infection induces the expression of tthb178 in a CRP-dependent manner in vivo (69). Upon phage infection, T. thermophilus CRP up-regulates the transcription of not only tthb178 but also a variety of clustered regularly interspaced short pal- indromic repeat (CRISPR)-associated genes, so-called cas genes (27), which have been implicated as the components of a host defense system against invading foreign replicons (70,71). The 30–50 exonuclease activity of Tth ExoI may also be utilized for the bacterial host defense system. SUPPLEMENTARY DATA Supplementary Data are available at NAR Online. ACKNOWLEDGEMENTS The authors thank Dr Akeo Shinkai and Yashihiro Agari for their valuable discussion on this study. They also thank Dr Yoshinori Koyama for providing pHG305 plasmid and Naoko Aoki for her excellent help in the construction of the plasmid for gene disruption. FUNDING Funding for open access charge: Ministry of Education, Culture, Sports, Science and Technology of Japan, Grant-in-Aid for Scientiﬁc Research (grant numbers 20570131 to R.M., 19770083 to N.N. and 20870042 to K.F.). Conﬂict of interest statement. None declared. Nucleic Acids Research, 2010, Vol. 38, No. 17 5703 REFERENCES 1. Crow,Y.J., Hayward,B.E., Parmar,R., Robins,P., Leitch,A., Ali,M., Black,D.N., van Bokhoven,H., Brunner,H.G., Hamel,B.C. et al. 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