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Journal of Bacteriology, April 2009, p. 2783-2794, Vol. 191, No. 8
0021-9193/09/$08.00+0 doi:10.1128/JB.01802-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Borrelia burgdorferi bba74 Is Expressed Exclusively during Tick Feeding and Is Regulated by Both Arthropod- and Mammalian Host-Specific Signals
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Vishwaroop B. Mulay,1,
Melissa J. Caimano,1,*
Radha Iyer,2
Star Dunham-Ems,1
Dionysios Liveris,2
Mary M. Petzke,2
Ira Schwartz,2 and
Justin D. Radolf1,3
Departments of Medicine,1 Genetics and Developmental Biology, University of Connecticut Health Center, Farmington, Connecticut 06030,3 Department of Microbiology and Immunology, New York Medical College, Valhalla, New York 105952
Received 22 December 2008/ Accepted 4 February 2009
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 the Borrelia 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, and ospE) and their proteins. In vitro, transcripts for bba74 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-type B. burgdorferi grown in vitro, in conjunction with expression analysis of DMC-cultivated wild-type and rpoS mutant spirochetes, revealed that, like ospA, bba74 is transcribed by 
70 and is subject to RpoS-mediated repression within the mammalian host. A series of experiments utilizing wild-type and rpoS mutant spirochetes was conducted to determine the transcriptional and translational profiles of bba74 during the tick-mouse cycle. Results from these studies revealed (i) that bba74 is transcribed by 70 exclusively during the larval and nymphal blood meals and (ii) that transcription of bba74 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 of B. 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 of ospA and ospC, 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 as B. burgdorferi transitions between its arthropod vector and mammalian host.
* Corresponding author. Mailing address: Department of Medicine, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030-3715. Phone: (860) 679-8390. Fax: (860) 679-1358. E-mail: mcaima{at}up.uchc.edu
Published ahead of print on 13 February 2009.
Supplemental material for this article may be found at http://jb.asm.org/.
V.B.M. and M.J.C. contributed equally to this research.
Journal of Bacteriology, April 2009, p. 2783-2794, Vol. 191, No. 8
0021-9193/09/$08.00+0 doi:10.1128/JB.01802-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
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