Dramatic alteration of surface lipoprotein profiles is a key strategy that Borrelia burgdorferi, the Lyme disease pathogen, has evolved for adapting to the diverse environments of arthropod and mammalian hosts. Several of these differentially expressed lipoproteins have been shown to play important roles in the enzootic cycle of B. burgdorferi. The BBA05 protein is a previously identified putative lipoprotein (P55 or S1 antigen) that elicits antibody responses in mammals. Recent microarray analyses indicate that the BBA05 gene is differentially expressed by many environmental factors, including temperature. However, the role of the BBA05 protein in the life cycle of B. burgdorferi has not been elucidated. Here we show that expression of the BBA05 gene was exclusively induced in feeding nymphal ticks during the spirochetal transmission from ticks to mammals. Upon generating a BBA05 mutant in an infectious strain of B. burgdorferi, we showed that the BBA05 mutant remained capable of establishing infection in mice, being acquired by ticks, persisting through tick molting, and reinfecting new mammalian hosts. These results indicate that, despite being a highly conserved and regulated antigen, the BBA05 protein has a nonessential role in the transmission cycle of B. burgdorferi, at least in the animal model.Borrelia burgdorferi, the causative agent of Lyme disease, has an astonishing number of lipoprotein genes (ϳ150) in its genome (over 10% of the total genome) (9, 21); many of these lipoprotein genes are differentially expressed during the enzootic cycle of B. burgdorferi in the two diverse hosts, an arthropod tick (e.g., Ixodes scapularis) vector and a mammalian host (e.g., white-footed mice) (46,50,54,59). A body of evidences indicates that these differentially regulated lipoproteins are important for B. burgdorferi's maintenance in its natural cycle. For example, OspA, an outer surface lipoprotein that is expressed chiefly in unfed ticks, functions as an adhesin essential for spirochetal survival in the tick vector (12,41,42,47,56,70). The outer surface lipoprotein OspC, on the other hand, is induced when ticks feed, concomitant with the downregulation of OspA (17,24,55,56). OspC is not required for B. burgdorferi replication in the tick vector but is essential for spirochetes to establish infection in the mammalian host (26,51,62,63). Although controversial, it has been proposed that OspC may also contribute to spirochetal transmission from ticks to mice (16,26,48,51,63). In addition to OspA and OspC, several other lipoproteins, such as DbpB/A, BBK32, BB0365, and BBA64, were also shown to play a role in spirochetal colonization either in the tick vector or in the mammalian host (2,18,37,45,57,58,71). Thus, studying the functions and regulation of B. burgdorferi lipoproteins is significant to our understanding of how B. burgdorferi adapts to diverse hosts in its natural cycle.In the past few years, we and others identified a regulatory pathway, the Rrp2-RpoN-RpoS pathway (also called the 54 -S sigma factor casca...