Molecular Microbiology

Johnston, Jason W.; Zaleski, Anthony; Allen, Simon; Mootz, Joe M.; Armbruster, David; Gibson, Bradford W.; Apicella, Michael A.; Munson, Robert S.

doi: 10.1111/j.1365-2958.2007.05890.xpmid: 17880422

Virulence of nontypeable Haemophilus influenzae (NTHi) is dependent on the decoration of lipooligosaccharide with sialic acid. This sugar must be derived from the host, as NTHi cannot synthesize sialic acids. NTHi can also use sialic acid as a carbon source. The genes encoding the sialic acid transporter and the genes encoding the catabolic activities are localized to two divergently transcribed operons, the siaPT operon and the nan operon respectively. In this study, we identified SiaR as a repressor of sialic acid transport and catabolism in NTHi. Inactivation of siaR resulted in the unregulated expression of the genes in both operons. Unregulated catabolism of sialic acid in the siaR mutant resulted in the reduction of surface sialylation and an increase in serum sensitivity. In addition to SiaR‐mediated repression, CRP, the cAMP receptor protein, was shown to activate expression of the siaPT operon but not the nan operon. We describe a model in which SiaR and CRP work to modulate intracellular sialic acid levels. Our results demonstrate the importance of SiaR‐mediated regulation to balance the requirement of surface sialylation and the toxic accumulation of intracellular sialic acid.

Gaballa, Ahmed; Helmann, John D.

doi: 10.1111/j.1365-2958.2007.05905.xpmid: 17725565

When iron is scarce, Bacillus subtilis expresses genes involved in the synthesis and uptake of the siderophore bacillibactin (BB) and uptake systems to pirate other microbial siderophores. Here, we demonstrate that transcriptional induction of the feuABCybbA operon, encoding the Fe–BB uptake system, is mediated by Btr (formerly YbbB), which is encoded by the immediately upstream gene. Btr contains an AraC‐type DNA binding domain fused to a substrate binding protein (SBP) domain related to FeuA, the SBP for Fe–BB uptake. When cells are iron‐limited, the Fur‐mediated repression of btr is relieved and Btr binds to a conserved direct repeat sequence adjacent to feuA to activate transcription. If BB is present, Btr further activates feuA expression. Btr binds with high affinity to both apo–BB and Fe–BB, and the resulting complex displays a significantly increased efficacy as a transcriptional activator relative to Btr alone. Btr can also activate transcription in response to the structurally similar siderophore enterobactin, although genetic analyses indicate that the two siderophores make distinct interactions with the Btr substrate binding domain. Thus, the FeuABC transporter is optimally expressed under conditions of iron starvation, when Fur‐mediated repression is relieved, and in the presence of its cognate substrate.

Ma, Liang; Jensen, Jørgen S.; Myers, Leann; Burnett, Judy; Welch, Mary; Jia, Qiuyao; Martin, David H.

doi: 10.1111/j.1365-2958.2007.05911.xpmid: 17784912

Mycoplasma genitalium, a human pathogen associated with sexually transmitted diseases, is unique in that it has smallest genome of any known free‐living organism. The goal of this study was to investigate if and how M. genitalium uses a minimal genome to generate genetic variations. We analysed the sequence variability of the third gene (MG192 or mgpC) of the M. genitalium MgPa adhesion operon, demonstrated that the MG192 gene is highly variable among and within M. genitalium strains in vitro and in vivo, and identified MG192 sequence shifts in the course of in vitro passage of the G37 type strain and in sequential specimens from an M. genitalium‐infected patient. In order to establish the origin of the MG192 variants, we examined nine genomic loci containing partial copies of the MgPa operon, known as MgPar sequences. Our analysis suggests that the MG192 sequence variation is achieved by recombination between the MG192 expression site and MgPar sequences via gene cross‐over and, possibly, also by gene conversion. It appears plausible that M. genitalium has the ability to generate unlimited variants from its minimized genome, which presumably allows the organism to adapt to diverse environments and/or to evade host defences by antigenic variation.

Ulrich, Martina; Bastian, Mike; Cramton, Sarah E.; Ziegler, Katrin; Pragman, Alexa A.; Bragonzi, Alessandra; Memmi, Guido; Wolz, Christiane; Schlievert, Patrick M.; Cheung, Ambrose; Döring, Gerd