Abstract
The Znu system, encoded for by znuABC, can be found in multiple genera of bacteria and has been shown to be responsible for the import of zinc under low zinc conditions. Although this high-affinity uptake system is known to be important for both growth and/or pathogenesis in bacteria, it has not been functionally characterized in a plant-associated bacterium. A single homologue of this system has been identified in the plant endosymbiont, Sinorhizobium meliloti, while two homologous systems were found in the destructive citrus pathogen, Candidatus Liberibacter asiaticus. To understand the role of these protein homologues, a complementation assay was devised allowing the individual genes that comprise the system to be assayed independently for their ability to reinstate a partially-inactivated Znu system. Results from the assays have demonstrated that although all of the genes from S. meliloti were able to restore activity, only one of the two Ca. Liberibacter asiaticus encoded gene clusters contained genes that were able to functionally complement the system. Additional analysis of the gene clusters reveals that distinct modes of regulation may also exist between the Ca. Liberibacter asiaticus and S. meliloti import systems despite the intracellular-plant niche common to both of these bacteria.
Highlights
The ability to import zinc is critical for many species of bacteria because of its use as an enzymatic cofactor or structural element in many cellular proteins [1]
The Znu system has been shown to be important for both growth and/or pathogenesis in several bacterium such as Synechocystis [6], Pasteurella multocida [7], and Salmonella enterica [8], it has not been characterized in plant-associated bacteria
The arrangement of the genes in the S. meliloti cluster resembles that of E. coli with the exception of the zinc uptake regulator, zur, which is located immediately upstream of znuB in S. meliloti but is approximately 2.2 Mb downstream of znuA in E. coli (Fig. 1)
Summary
The ability to import zinc is critical for many species of bacteria because of its use as an enzymatic cofactor or structural element in many cellular proteins [1] Since zinc is both highly charged and hydrophilic, it cannot cross the bacterial membrane via passive diffusion [2] and must gain entrance into the bacterial cells though the actions of either non-specific or specific transport systems [3]. The Znu (zinc uptake) system has a high affinity for zinc and was discovered in Escherichia coli during a transposon screen with lacZ gene cluster fusions [5] This system has since been classified as a member of the ATP-binding cassette (ABC) transporter family and is composed of three protein products: ZnuA, ZnuB, and ZnuC. The Znu system has been shown to be important for both growth and/or pathogenesis in several bacterium such as Synechocystis [6], Pasteurella multocida [7], and Salmonella enterica [8], it has not been characterized in plant-associated bacteria
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