Regulation and Properties of PstSCAB, a High-Affinity, High-Velocity Phosphate Transport System of<i>Sinorhizobium meliloti</i>
Bibliographic record
Abstract
The properties and regulation of the pstSCAB-encoded Pi uptake system from the alfalfa symbiont Sinorhizobium meliloti are reported. We present evidence that the pstSCAB genes and the regulatory phoUB genes are transcribed from a single promoter that contains two PhoB binding sites and that transcription requires PhoB. S. meliloti strain 1021 (Rm1021) and its derivatives were found to carry a C deletion frameshift mutation in the pstC gene (designated pstC1021) that severely impairs activity of the PstSCAB Pi transport system. This mutation is absent in RCR2011, the parent of Rm1021. Correction of the pstC1021 mutation in Rm1021 by site-directed mutagenesis revealed that PstSCAB is a Pi-specific, high-affinity (Km, 0.2 microM), high-velocity (Vmax, 70 nmol/min/mg protein) transport system. The pstC1021 allele was shown to generate a partial pho regulon constitutive phenotype, in which transcription is activated by PhoB even under Pi-excess conditions that render PhoB inactive in a wild-type background. The previously reported symbiotic Fix- phenotype of phoCDET mutants was found to be dependent on the pstC1021 mutation, as Rm1021 phoCDET mutants formed small white nodules on alfalfa that failed to reduce N2, whereas phoCDET mutant strains with a corrected pstC allele (RmP110) formed pink nodules on alfalfa that fixed N2 like the wild type. Alfalfa root nodules formed by the wild-type RCR2011 strain expressed the low-affinity orfA-pit-encoded Pi uptake system and neither the pstSCAB genes nor the phoCDET genes. Thus, metabolism of alfalfa nodule bacteroids is not Pi limited.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".