SUMMARY Establishment of a community is considered to be essential for microbial growth and survival in the human oral cavity. Biofilm communities have increased resilience to physical forces, antimicrobial agents, and nutritional variations. Specific cell-to-cell adherence processes, mediated by adhesin-receptor pairings on respective microbial surfaces, are able to direct community development. These interactions co-localize species in mutually beneficial relationships, such as streptococci, veillonellae, Porphyromonas gingivalis and Candida albicans. In transition from the planktonic mode of growth to a biofilm community, microorganisms undergo major transcriptional and proteomic changes. These occur in response to sensing of diffusible signals, such as autoinducer molecules, and to contact with host tissues or other microbial cells. Underpinning many of these processes are intracellular phosphorylation events that regulate a large number of microbial interactions relevant to community formation and development.

The Lyme disease spirochete, Borrelia burgdorferi, encodes a novel type of DNA-binding protein named EbfC. Orthologs of EbfC are encoded by a wide range of bacterial species, so characterization of the borrelial protein has implications that span the eubacterial kingdom. The present work defines the DNA sequence required for high-affinity binding by EbfC to be the 4 bp broken palindrome GTnAC, where ‘n’ can be any nucleotide. Two high-affinity EbfC-binding sites are located immediately 5′ of B. burgdorferi erp transcriptional promoters, and binding of EbfC was found to alter the conformation of erp promoter DNA. Consensus EbfC-binding sites are abundantly distributed throughout the B. burgdorferi genome, occurring approximately once every 1 kb. These and other features of EbfC suggest that this small protein and its orthologs may represent a distinctive type of bacterial nucleoid-associated protein. EbfC was shown to bind DNA as a homodimer, and site-directed mutagenesis studies indicated that EbfC and its orthologs appear to bind DNA via a novel α-helical ‘tweezer’-like structure.

Successful infection of a host requires that the invading pathogen control its production of virulence determinants. The infectious agent must sense its environment and respond by increasing production of appropriate factors and repressing production of unnecessary ones. These features are especially critical for vector-borne pathogens, which must not only efficiently infect two extremely different host types but also be transmitted back and forth between hosts. Deciphering the regulatory pathways used by pathogens to control production of infection-associated proteins provides significant insight into the infectious nature of those organisms. Moreover, regulatory factors are attractive candidates for development of novel preventative and curative therapies.The spirochetal bacterium Borrelia burgdorferi, the agent of Lyme disease, is an excellent model organism for the study of gene regulation by a vector-borne pathogen. B. burgdorferi is genetically tractable, and its natural mammal-tick infectious cycle can be replicated in the laboratory. In addition, infection by B. burgdorferi is a significant cause of human morbidity, being the most commonly reported vector-borne disease in the United States and many other parts of the world (51, 55, 56).B. burgdorferi Erp lipoproteins are produced throughout mammalian infection but are largely repressed during colonization of vector ticks (10,31,48,49). Erp synthesis is greatly enhanced when B. burgdorferi is transmitted from a feeding tick into a warm-blooded host. Regulation of Erp protein production is controlled at the level of transcription (6). Erp proteins are located in the bacterial outer membrane and are exposed to the external environment (25,32,41). Known functions of Erp proteins include binding of host plasmin(ogen), laminin, and the complement regulators factor H and factor H-related proteins 1, 2, and 5 (2,3,11,12,34,37,40,45,59). These functions indicate roles for Erp proteins in host adherence, dissemination, and resistance to the alternative pathway of complement-mediated killing. Borrelial erp genes are located in mono-or bicistronic operons on extrachromosomal cp32 prophages, most of which replicate autonomously as circular episomes (24,60,63,64,72). Individual Lyme spirochetes naturally contain numerous different cp32 elements, each with a unique erp locus, and therefore produce multiple, distinct Erp surface proteins. A bacterium simultaneously expresses its entire repertoire of Erp proteins (26).A highly conserved DNA region immediately 5= of all erp promoters, the erp operator, is required for regulation of erp transcription (see Fig. 1) (6,10,64). Two erp operator-binding proteins have been identified, and their binding sites have been characterized: BpaB (borrelial plasmid ParB analogue) and EbfC (erp-binding factor, chromosomal) (4, 13, 52). BpaB binds with high affinity to a 5-bp sequence within the erp operator (13; C. A. Adams, unpublished). Binding of one BpaB protein to that sequence then facilitates binding of additional BpaB molecules al...

Borrelia burgdorferi produces Erp outer surface proteins throughout mammalian infection, but represses their synthesis during colonization of vector ticks. A DNA region 5′ of the start of erp transcription, Operator 2, was previously shown to be essential for regulation of expression. We now report identification and characterization of a novel erp Operator 2-binding protein, which we named BpaB. erp operons are located on episomal cp32 prophages, and a single bacterium may contain as many as 10 different cp32s. Each cp32 family member encodes a unique BpaB protein, yet the three tested cp32-encoded BpaB alleles all bound to the same DNA sequence. A 20-bp region of erp Operator 2 was determined to be essential for BpaB binding, and initial protein binding to that site was required for binding of additional BpaB molecules. A 36-residue region near the BpaB carboxy terminus was found to be essential for high-affinity DNA-binding. BpaB competed for binding to erp Operator 2 with a second B. burgdorferi DNA-binding protein, EbfC. Thus, cellular levels of free BpaB and EbfC could potentially control erp transcription levels.