and then processed into 16S, 23S and 5S rRNA [11, 13]. This organization permits synthesis of equimolar amounts of each rRNA species. In E. coli, rRNA synthesis involves the transcription factor DksA . It is negatively regulated by (p)ppGpp (guanosine-3′-diphosphate-5′-triphosphate and guanosine-3′,5′-bisphosphate, collectively), a global regulator involved in bacterial adaptation to many environmental stresses, and positively regulated by the concentration of the initiating nucleoside triphosphates acting in trans on the P1 and P2 rRNA promoters . The other major mechanism to control rRNA synthesis in E. coli is growth rate-dependent control . Under this (p)ppGpp-independent control mechanism, ribosome concentration in each cell is proportional to growth rate. The B. burgdorferi chromosome contains a single 16S rRNA gene and two tandem sets of 23S and 5S rRNA genes located at nt435201-446118, as well as genes encoding transfer tRNAs for alanine (tRNAAla) and isoleucine (tRNAIle) [10, 15, 16] (Figure 1). All these genes except tRNAIle are present in the same orientation on the chromosome. Not only are patterns of transcription and regulation Trichostatin A order of rRNA genes uncharacterized in B. burgdorferi, but there is little information as to
whether rRNA synthesis in this bacterium is regulated by the stringent response, by growth rate, or by some other mechanism. We previously found that B.
burgdorferi N40 co-cultured with tick cells down-modulated its levels of (p)ppGpp and decreased rel Bbu expression while growing more slowly than in Barbour-Stoenner-Kelly (BSK)-H medium . This simultaneous decrease in (p)ppGpp and growth rate was associated with down-modulation of 16S rRNA , and suggested that growth rate but not (p)ppGpp or the stringent response regulated Mirabegron rRNA levels in B. burgdorferi. A B. burgdorferi 297 Δ rel Bbu deletion mutant lost both the ability to synthesize (p)ppGpp and to reach stationary phase cell densities as high as those of its wild-type parent even though the parent and the mutant multiplied at similar rates during exponential phase of growth . Figure 1 Transcriptional organization of B. burgdorferi B31 chromosomal region containing rRNA genes [10, 15, 16]. Short arrows indicate the position of primers from Table 1 used for analysis of rRNA expression in B. burgdorferi. We have now examined both the organization of transcription of B. burgdorferi rRNA and the influence of growth phase and the stringent response on rRNA synthesis. This information is especially critical to improving our understanding of the ability of B. burgdorferi to shift between the rapid growth of acute mammalian and arthropod infection and slow growth during persistence in these hosts [3, 20, 21].