A conservative amino acid change alters the function of BosR, the redox regulator ofBorrelia burgdorferi.

Borrelia burgdorferi, the aetiologic agent of Lyme disease, modulates gene expression in response to changes imposed by its arthropod vector and mammalian hosts. As reactive oxygen species (ROS) are known to vary in these environments, we asked howB. burgdorferiresponds to oxidative stress. TheB. burgdorferigenome encodes a PerR homologue (recently designated BosR) that represses the oxidative stress response in other bacteria, suggesting a similar function inB. burgdorferi. When we tested the sensitivity ofB. burgdorferito ROS, one clonal non-infectiousB. burgdorferiisolate exhibited hypersensitivity tot-butyl hydroperoxide when compared with infectiousB. burgdorferiand other non-infectious isolates. Sequence analysis indicated that the hypersensitive non-infectious isolatesbosRallele contained a single nucleotide substitution, converting an arginine to a lysine (bosRR39K). Mutants inbosRR39Kexhibited an increase in resistance to oxidative stressors when compared with the parental non-infectious strain, suggesting that BosRR39K functioned as a repressor. Complementation withbosRR39KandbosRresulted in differential sensitivity tot-butyl hydroperoxide, indicating that these alleles are functionally distinct. In contrast to BosR, BosRR39K did not activate transcription of anapApromoter– lacZreporter inEscherichia colinor bind thenapApromoter/operator domain. However, we found that both BosR and BosRR39K bound to the putative promoter/operator region of superoxide dismutase (sodA). In addition, we determined that cells lacking BosRR39K synthesized fourfold greater levels of the decorin binding adhesin DbpA suggesting that BosRR39K regulates genes unrelated to oxidative stress. Based on these data, we propose that the single amino acid substitution, R39K, dramatically alters the activity of BosR by altering its ability to bind DNA at target regulatory sequences. [ABSTRACT FROM AUTHOR]