A number of bacterial metal transporters belong to the ABC transporter family. To better understand the structural determinants of metal selectivity of one such transporter, we previously determined the structure of the periplasmic domain of a zinc transporter, ZnuA, from Synechocystis 6803 and found that ZnuA binds zinc via three histidines. Unique to these ABC zinc transporters, ZnuA has a highly charged and mobile loop that protrudes from the protein in the vicinity of the metal binding site that we had suggested might facilitate zinc acquisition. To further examine the function of this loop, the structure and zinc binding properties of two ZnuA variants were determined. When the loop is entirely deleted, zinc still binds to the three histidines. However, unlike what was suggested from the structure of a similar solute binding protein, TroA, release of zinc occurs concomitantly with large conformational changes in two of the three chelating histidines. These structural results combined with isothermal titration calorimetry data demonstrate that there are at least two classes of zinc binding sites: the high-affinity site in the cleft between the two domains and at least one additional site on the flexible loop. This loop has approximately 100-fold weaker affinity for zinc than the high-affinity zinc binding site, and its deletion does not affect the high-affinity site. From these results, we suggest that this region might be a sensor for high periplasmic levels of zinc.

The modern age of metagenomics has delivered unprecedented volumes of data describing the genetic and metabolic diversity of bacterial communities, but it has failed to provide information about coincident cellular morphologies. Much like metabolic and biosynthetic capabilities, morphology comprises a critical component of bacterial fitness, molded by natural selection into the many elaborate shapes observed across the bacterial domain. In this essay, we discuss the diversity of bacterial morphology and its implications for understanding both the mechanistic and the adaptive basis of morphogenesis. We consider how best to leverage genomic data and recent experimental developments in order to advance our understanding of bacterial shape and its functional importance.

Bacteria exhibit a rich diversity of morphologies. Within this diversity, there is a uniformity of shape for each species that is replicated faithfully each generation, suggesting that bacterial shape is as selectable as any other biochemical adaptation. We describe the spatiotemporal mechanisms that target peptidoglycan synthesis to different subcellular zones to generate the rod-shape of model organisms Escherichia coli and Bacillus subtilis. We then demonstrate, using the related genera Caulobacter and Asticcacaulis as examples, how the modularity of the core components of the peptidoglycan synthesis machinery permits repositioning of the machinery to achieve different growth modes and morphologies. Finally, we highlight cases in which the mechanisms that underlie morphological evolution are beginning to be understood, and how they depend upon the expansion and diversification of the core components of the peptidoglycan synthesis machinery.

Cyanobacteria account for a significant percentage of aquatic primary productivity even in areas where the concentrations of essential micronutrients are extremely low. To better understand the mechanism of iron selectivity and transport, the structure of the solute binding domain of an ATP binding cassette iron transporter, FutA1, was determined in the presence and absence of iron. The iron ion is bound within the "C-clamp" structure via four tyrosine and one histidine residues. There are extensive interactions between these ligating residues and the rest of the protein such that the conformations of the side chains remain relatively unchanged as the iron is released by the opening of the metal binding cleft. This is in stark contrast to the zinc-binding protein, ZnuA, where the domains of the metalbinding protein remain relatively fixed, whereas the ligating residues rotate out of the binding pocket upon metal release. The rotation of the domains in FutA1 is facilitated by two flexible ␤-strands running along the back of the protein that act like a hinge during domain motion. This motion may require relatively little energy since total contact area between the domains is the same whether the protein is in the open or closed conformation. Consistent with the pH dependence of iron binding, the main trigger for iron release is likely the histidine in the ironbinding site. Finally, neither FutA1 nor FutA2 binds iron as a siderophore complex or in the presence of anions, and both preferentially bind ferrous over ferric ions.Bioavailable iron is a limiting nutrient for primary production in large areas of the oceans. This concentration of free iron in aquatic environments is dynamic and varies greatly depending upon the local environment. Fe 3ϩ is notoriously insoluble in water at neutral pH values, whereas Fe 2ϩ is very soluble but highly susceptible to oxidation by atmospheric oxygen. Microbes play a large role in the cycling of iron between the ferric and ferrous forms and generally reduce ferric iron under anaerobic conditions by using it as a final electron acceptor. Conversely, microbes can oxidize ferrous iron under aerobic conditions when other compounds such as nitrate are the final electron acceptors. Organisms can import either form of iron. A number of bacteria, algae, and Cyanobacteria increase the bioavailability of ferric iron through the secretion of organic molecules, such as siderophores, into the extracellular environment. These compounds have exceptionally high binding affinities for iron (association constants of ϳ10 MϪ1 ) and essentially scavenge ferric iron from the extracellular environment before the organism imports the entire complex into the cell (e.g. Ref. 1). In some cases these siderophores may actually facilitate a photochemical reduction of the bound ferric ion (2). Alternatively, ferric iron can be locally reduced to the more soluble ferrous form and imported directly. This latter can be accomplished by the organisms itself as is the case with some algal species that use ferric chel...

Agrobacterium tumefaciens , a significant biotechnological tool for production of transgenic plant lines, is highly resistant to a wide variety of antibiotics, posing challenges for various applications. One challenge is the efficient elimination of A. tumefaciens from transformed plant tissue without using levels of antibiotics that are toxic to the plants.