Borrelia burgdorferi bba74 Is Expressed Exclusively during Tick Feeding and Is Regulated by Both Arthropod- and Mammalian Host-Specific Signals

Although BBA74 initially was described as a 28-kDa virulence-associated outer-membrane-spanning protein with porin-like function, subsequent studies revealed that it is periplasmic and downregulated in mammalian host-adapted spirochetes. To further elucidate the role of this protein in theBorrelia burgdorferi tick-mammal cycle, we conducted a thorough examination of its expression profile in comparison with the profiles of three well-characterized, differentially expressed borrelial genes (ospA ,ospC , andospE ) and their proteins. In vitro, transcripts forbba74 were expressed at 23°C and further enhanced by a temperature shift (37°C), whereas BBA74 protein diminished at elevated temperatures; in contrast, neither transcript nor protein was expressed by spirochetes grown in dialysis membrane chambers (DMCs). Primer extension of wild-typeB. burgdorferi grown in vitro, in conjunction with expression analysis of DMC-cultivated wild-type andrpoS mutant spirochetes, revealed that, likeospA ,bba74 is transcribed by σ70 and is subject to RpoS-mediated repression within the mammalian host. A series of experiments utilizing wild-type andrpoS mutant spirochetes was conducted to determine the transcriptional and translational profiles ofbba74 during the tick-mouse cycle. Results from these studies revealed (i) thatbba74 is transcribed by σ70 exclusively during the larval and nymphal blood meals and (ii) that transcription ofbba74 is bracketed by RpoS-independent and -dependent forms of repression that are induced by arthropod- and mammalian host-specific signals, respectively. Although loss of BBA74 does not impair the ability ofB. burgdorferi to complete its infectious life cycle, the temporal compartmentalization of this gene's transcription suggests that BBA74 facilitates fitness of the spirochete within a narrow window of its tick phase. A reexamination of the paradigm for reciprocal regulation ofospA andospC , performed herein, revealed that the heterogeneous expression of OspA and OspC displayed by spirochete populations during the nymphal blood meal results from the intricate sequence of transcriptional and translational changes that ensue asB. burgdorferi transitions between its arthropod vector and mammalian host.