Isolation of Escherichia coli mutants with changed regulation of uracil uptake

Conservation of the Pho regulon in Pseudomonas fluorescens Pf0-1

The Pho regulon integrates the sensing of environmental inorganic phosphate (Pi) availability with coregulation of gene expression, mediating an adaptive response to Pi limitation. Many aspects of the Pho regulon have been addressed in studies of Escherichia coli; however, it is unclear how transferable this knowledge is to other bacterial systems. Here, we report work to discern the conservation of the Pho regulon in Pseudomonas fluorescens Pf0-1. We demonstrate by mutational studies that PhoB/PhoR and the Pst system have conserved functions in the regulation of Pi-induced phosphatase activities, as well as expression of other Pi-regulated genes. A genetic screen was carried out to isolate factors that affect Pho-regulated phosphatase activity. We identified the Pho-regulated phosphatases PhoX and PhoD and present evidence that these enzymes are exported via the Tat system. The phoX and phoD genes were shown to be members of the Pho regulon by reverse transcription-PCR, as well as by functional assessment of putative PhoB binding sites (Pho boxes). Our data also suggested that at least one other non-Tat-secreted Pho-regulated phosphatase exists. From the genetic screen, numerous siderophore mutants that displayed severe defects in Pho-activated phosphatase activity were isolated. Subsequently, iron was shown to be important for modulating the activity of Pho-regulated phosphatases, but it does not regulate this activity at the level of transcription. We also identify and demonstrate a novel role in siderophore production and Pho-regulated phosphatase activity for ApaH, the hydrolase for the nucleotide-signaling molecule AppppA. Finally, numerous mutations in multiple cellular pathways were recovered that may be required for maximal induction of the Pho regulon under Pi-limiting conditions.

Specificity and control of uptake of purines and other compounds in Bacillus subtilis

Certain nucleotides control adaptation to changing nutrition or differentiation (sporulation) resulting from a general nutritional deficiency. To maintain the adaptation or differentiation process, once it has started, it may have been important for cells to evolve several independent and metabolically controllable systems enabling the uptake and metabolism of various nucleic acid bases or nucleosides. We have analyzed the cellular reactions with these compounds by measuring both their effect on growth and their uptake in appropriately chosen auxotrophic and uptake mutants. We have found one uptake system for guanine and hypoxanthine, another one for guanosine and inosine, and three other systems for adenine, adenosine, and uracil. The uptake systems of guanine-hypoxanthine and guanosine-inosine are inhibited by the stringent response to amino acid deprivation (increase of guanosine 5'-diphosphate-3'-diphosphate), but they do not depend on the concentration of GTP, which decreases during sporulation. In contrast, the uptake of Ura depends on the presence of GTP, regardless of whether a GTP decrease was produced by the stringent response or otherwise. This was the only uptake system whose decrease was always correlated with the onset of sporulation. The uptake of other compounds, e.g., alpha-methylglucoside and alpha-aminoisobutyric acid, decreased under some, but not all, sporulation conditions.