Phosphate starvation response controls genes required to synthesize the phosphate analog arsenate

Abstract

Environmental arsenic poisoning affects roughly 200 million people worldwide. The toxicity and mobility of arsenic in the environment is significantly influenced by microbial redox reactions, with arsenite (As ) being more toxic than arsenate (As ). Microbial oxidation of As to As is known to be regulated by the AioXSR signal transduction system and viewed to function for detoxification or energy generation. Here, we show that As oxidation is ultimately regulated by the phosphate starvation response (PSR), requiring the sensor kinase PhoR for expression of the As oxidase structural genes aioBA. The PhoRB and AioSR signal transduction systems are capable of transphosphorylation cross-talk, closely integrating As oxidation with the PSR. Further, under PSR conditions, As significantly extends bacterial growth and accumulates in the lipid fraction to the apparent exclusion of phosphorus. This could spare phosphorus for nucleic acid synthesis or triphosphate metabolism wherein unstable arsenic esters are not tolerated, thereby enhancing cell survival potential. We conclude that As oxidation is logically part of the bacterial PSR, enabling the synthesis of the phosphate analog As to replace phosphorus in specific biomolecules or to synthesize other molecules capable of a similar function, although not for total replacement of cellular phosphate.

Transcriptomics analysis defines global cellular response ofAgrobacterium tumefaciens5A to arsenite exposure regulated through the histidine kinases PhoR and AioS

“…Grouping the genes according to function illustrated a truly global response, involving the perturbation of a wide range of cellular processes (Table and Supporting Information Table 1). Differences in regulation included phenomena that were expected based on prior work (Kashyap et al ., ; Kang et al ., ; ; Wang et al ., ), such as upregulation of aio genes (twofold to 87‐fold increase), arsenic resistance (threefold to 110‐fold) and pho/pst1 locus genes (twofold to 555‐fold; see Fig. ).…”

“…The different regulatory patterns of the aio genes observed for the wild type and mutants offer new insight into the regulation of As III oxidation (Table ). Several important observations should be noted: (i) in wild type cells, aioX , aioS and aioR were all similarly induced (~twofold to threefold), whereas aioB and aioA transcript abundance was an order of magnitude greater (Table ); (ii) aioX induction by As III appears uninfluenced by AioS but requires PhoR (consistent with Wang et al ., ); (iii) aioB, aioA, aioR and aioS require both AioS and PhoR (Table ); (iv) aioB and aioA are upregulated to different extents in the wild type cell (~threefold difference), illustrating that transcription of these genes is somehow uncoupled, although this difference largely disappears in either mutant (Supporting Information Table 1); and (v) in the absence of As III , aioB expression, but not aioA , was significantly reduced in both mutants (about the same level), indicating that both AioS and PhoR are somehow necessary for aioB expression in the absence of As III (Table ).…”

“…Comparing the phoR mutant to the wildtype strain in −As III cells illustrates the pho/pst2 locus genes are significantly affected, whereas the pho/pst1 locus is not. By contrast, the pho/pst1 locus genes are highly upregulated by As III , and PhoR nevertheless exerts regulatory influence (Table ) (Wang et al ., ). In comparing pho/pst2 gene expression changes in the wildtype and aio S mutant, it appears that the As III ‐based repression of the pst/pho2 genes in the wild type strain is associated with AioS‐based signalling activity.…”

“…The PSR has yet to be characterized in either organism, but we note that PSR culture conditions used in the current study were very similar to those described for the ‘Late Phase’ expression patterns of H. arsenicoxydans (Cleiss‐Arnold et al ., ). A PSR scenario is less clear for Rhizobium NT‐26 (Andres et al ., ) because of the relatively high starting Pi concentration in the media used (1.25 mM), although this is similar to that used for A. tumefaciens strain GW4 (Wang et al ., ; ) and thus potentially may have translated to a PSR condition in the stationary phase cells used by Andres and colleagues (). To clarify this situation, the PSR needs to be characterized in these organisms so as to determine if there are substantive variations among these bacteria.…”

“…In comparing pho/pst2 gene expression changes in the wildtype and aio S mutant, it appears that the As III ‐based repression of the pst/pho2 genes in the wild type strain is associated with AioS‐based signalling activity. AioS is also essential for upregulating the As III oxidase genes ( aioBA ) required for synthesizing the phosphate analog, As V , that we hypothesized partially spares Pi in some cellular features (Wang et al ., ; ). This differential response and control circuitry of the pho/pst loci provides new insight as to their role in this bacterium.…”