Roberto Rebeil

Variation in Lipid A Structure in the Pathogenic Yersiniae

Important pathogens in the genus Yersinia include the plague bacillus Yersinia pestis and two enteropathogenic species, Yersinia pseudotuberculosis and Yersinia enterocolitica. A shift in growth temperature induced changes in the number and type of acyl groups on the lipid A of all three species. After growth at 37°C, Y. pestis lipopolysaccharide (LPS) contained the tetra‐acylated lipid IVA and smaller amounts of lipid IVA modified with C10 or C12 acyl groups, Y. pseudotuberculosis contained the same forms as part of a more heterogeneous population in which lipid IVA modified with C16:0 predominated, and Y. enterocolitica produced a unique tetra‐acylated lipid A. When grown at 21°C, however, the three yersiniae synthesized LPS containing predominantly hexa‐acylated lipid A. This more complex lipid A stimulated human monocytes to secrete tumour necrosis factor‐α, whereas the lipid A synthesized by the three species at 37°C did not. The Y. pestis phoP gene was required for aminoarabinose modification of lipid A, but not for the temperature‐dependent acylation changes. The results suggest that the production of a less immunostimulatory form of LPS upon entry into the mammalian host is a conserved pathogenesis mechanism in the genus Yersinia, and that species‐specific lipid A forms may be important for life cycle and pathogenicity differences.

The subunit structure and catalytic mechanism of the Bacillus subtilis DNA repair enzyme spore photoproduct lyase

The major DNA photoproduct of dormant, UV-irradiated Bacillus subtilis spores is the thymine dimer 5-thyminyl-5,6-dihydrothymine [spore photoproduct (SP)]. During spore germination, SP is reversed to two intact thymines in situ by the DNA repair enzyme SP lyase, an S-adenosylmethionine (S-AdoMet)-dependent iron-sulfur ([Fe-S]) protein encoded by the splB gene. In the present work, cross-linking, SDS/PAGE, and size exclusion chromatography revealed that SplB protein dimerized when incubated with iron and sulfide under anaerobic reducing conditions. SplB isolated under aerobic conditions generated an EPR spectrum consistent with that of a partially degraded [3Fe-4S] center, and reduction of SplB with dithionite shifted the spectrum to that of a [4Fe-4S] center. Addition of S-AdoMet to SplB converted some of the [4Fe-4S] centers to an EPR-silent form consistent with electron donation to S-AdoMet. HPLC and electrospray ionization MS analyses showed that SP lyase cleaved S-AdoMet to generate 5′-deoxyadenosine. The results indicate that (i) SP lyase is a homodimer of SplB; (ii) dimer formation is coordinated by a [4Fe-4S] center; and (iii) the reduced [4Fe-4S] center is capable of donating electrons to S-AdoMet to generate a 5′-adenosyl radical that is then used for the in situ reversal of SP. Thus, SP lyase belongs to the “radical SAM” superfamily of enzymes that use [Fe-S] centers and S-AdoMet to generate adenosyl radicals to effect catalysis. SP lyase is unique in being the first and only DNA repair enzyme known to function via this novel enzymatic mechanism.

Characterization of Late Acyltransferase Genes of Yersinia pestis and Their Role in Temperature-Dependent Lipid A Variation

Yersinia pestis is an important human pathogen that is maintained in flea-rodent enzootic cycles in many parts of the world. During its life cycle, Y. pestis senses host-specific environmental cues such as temperature and regulates gene expression appropriately to adapt to the insect or mammalian host. For example, Y. pestis synthesizes different forms of lipid A when grown at temperatures corresponding to the in vivo environments of the mammalian host and the flea vector. At 37 degrees C, tetra-acylated lipid A is the major form; but at 26 degrees C or below, hexa-acylated lipid A predominates. In this study, we show that the Y. pestis msbB (lpxM) and lpxP homologs encode the acyltransferases that add C12 and C(16:1) groups, respectively, to lipid IV(A) to generate the hexa-acylated form, and that their expression is upregulated at 21 degrees C in vitro and in the flea midgut. A Y. pestis deltamsbB deltalpxP double mutant that did not produce hexa-acylated lipid A was more sensitive to cecropin A, but not to polymyxin B. This mutant was able to infect and block fleas as well as the parental wild-type strain, indicating that the low-temperature-dependent change to hexa-acylated lipid A synthesis is not required for survival in the flea gut.

Induction of the Yersinia pestis PhoP-PhoQ Regulatory System in the Flea and Its Role in Producing a Transmissible Infection

Transmission of Yersinia pestis is greatly enhanced after it forms a bacterial biofilm in the foregut of the flea vector that interferes with normal blood feeding. Here we report that the ability to produce a normal foregut-blocking infection depends on induction of the Y. pestis PhoP-PhoQ two-component regulatory system in the flea. Y. pestis phoP-negative mutants achieved normal infection rates and bacterial loads in the flea midgut but produced a less cohesive biofilm both in vitro and in the flea and had a greatly reduced ability to localize to and block the flea foregut. Thus, not only is the PhoP-PhoQ system induced in the flea gut environment, but also this induction is required to produce a normal transmissible infection. The altered biofilm phenotype in the flea was not due to lack of PhoPQ-dependent or PmrAB-dependent addition of aminoarabinose to the Y. pestis lipid A, because an aminoarabinose-deficient mutant that is highly sensitive to cationic antimicrobial peptides had a normal phenotype in the flea digestive tract. In addition to enhancing transmissibility, induction of the PhoP-PhoQ system in the arthropod vector prior to transmission may preadapt Y. pestis to resist the initial encounter with the mammalian innate immune response.

Essential cysteine residues in Bacillus subtilis spore photoproduct lyase identified by alanine scanning mutagenesis.

Endospore-forming bacteria (Bacillus and Clostridium spp.) are highly ultraviolet (UV) resistant and repair UV-induced DNA damage in part using the spore-specific DNA repair enzyme spore photoproduct (SP) lyase. SP lyase in all known sporeformers contains four conserved cysteine residues; three absolutely conserved residues are located at the “Radical SAM” consensus (C91xxxC95xxC98), which presumably participates in [4Fe-4S] cluster formation. A fourth conserved cysteine, the function of which is unknown, is located at C141 in SP lyase from all Bacillus spp. sequenced to date. To probe the function of the fourth cysteine, each conserved cysteine in the B. subtilis SP lyase was systematically altered to alanine by site-directed mutagenesis. UV-visible spectroscopy of wild-type and mutant SP lyases indicated that C141 does not participate in [4Fe-4S] formation and redox chemistry; however, in vivo SP lyase activity was abolished in all mutants, indicating an essential role for C141 in SP lyase activity.

Exploratory research into pathogen surface interactions.

In this short-duration project the research team was able to achieve growth of both drinking water biofilms and monospecific biofilms of Legionella pneurnophila. Preliminary comparative proteomic analyses were carried out on planktonic and biofilm-associated Legionella. After delay for completion of permitting and review by the director of the National Institutes for Allergy and Infectious Disease, the Utah 112 strain of Francisella novicida was obtained and preliminary culture and comparative proteomic analyses were carried out. Comprehensive literature searches and data mining were carried out on all research topics.