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Borrelia burgdorferi , the causative agent of Lyme disease, traverses through vastly distinct environments between the tick vector and the multiple phases of the mammalian infection that requires genetic adaptation for the progression of pathogenesis. Borrelial gene expression is highly responsive to changes in specific environmental signals that initiate the RpoS regulon for mammalian adaptation, but the mechanism(s) for direct detection of environmental cues has yet to be identified. Secondary messenger cyclic adenosine monophosphate (cAMP) produced by adenylate cyclase is responsive to environmental signals, such as carbon source and pH, in many bacterial pathogens to promote virulence by altering gene regulation.  B. burgdorferi  encodes a single non-toxin class IV adenylate cyclase ( bb0723 ,  cyaB ). This study investigates  cyaB  expression along with its influence on borrelial virulence regulation and mammalian infectivity. Expression of  cyaB  was specifically induced with co-incubation of mammalian host cells that was not observed with cultivated tick cells suggesting that  cyaB  expression is influenced by cellular factor(s) unique to mammalian cell lines. The 3′ end of  cyaB  also encodes a small RNA, SR0623, in the same orientation that overlaps with  bb0722 . The differential processing of  cyaB  and SR0623 transcripts may alter the ability to influence function in the form of virulence determinant regulation and infectivity. Two independent  cyaB  deletion B31 strains were generated in 5A4-NP1 and ML23 backgrounds and complemented with the  cyaB  ORF alone that truncates SR0623,  cyaB  with intact SR0623, or  cyaB  with a mutagenized full-length SR0623 to evaluate the influence on transcriptional and posttranscriptional regulation of borrelial virulence factors and infectivity. In the absence of  cyaB , the expression and production of  ospC  was significantly reduced, while the protein levels for BosR and DbpA were substantially lower than parental strains. Infectivity studies with both independent  cyaB  mutants demonstrated an attenuated phenotype with reduced colonization of tissues during early disseminated infection. This work suggests that  B. burgdorferi  utilizes  cyaB  and potentially cAMP as a regulatory pathway to modulate borrelial gene expression and protein production to promote borrelial virulence and dissemination in the mammalian host.

The Borrelia burgdorferi Adenylate Cyclase, CyaB, Is Important for Virulence Factor Production and Mammalian Infection