Metal-Specific vs General Stress Genes in the Metal Fitness Atlas

1. 55% of Metal-Important Genes Are Metal-Specific

Of the 7,609 metal-important gene records with fitness matrix data across 24 organisms, 4,177 (54.9%) are metal-specific — they show significant fitness defects under metal stress but a <5% sick rate across 5,945 non-metal experiments. The remaining genes split into general sick (2,888, 38.0%) and metal+stress (544, 7.2%). This classification is robust across thresholds: at 2% sick rate, ~41% are metal-specific; at 10%, ~67% are.

Coverage note: 7 of 31 metal-tested organisms (ANA3, Dino, Keio, MR1, Miya, PV4, SB2B) could not be processed because their metal-important gene locusIds did not match the fitness matrix index format. The 24 included organisms account for 7,609 of 12,838 metal-important gene records (59.3%). The excluded organisms are taxonomically diverse and their absence is not expected to introduce systematic bias.

Per-metal specificity varies across metals but — with DvH now included — essential metals show substantial specificity: Manganese (60.6%), Molybdenum (60.5%), Tungsten (56.5%), Selenium (46.4%). Toxic metals range from 42-56% metal-specific. Iron is lowest at 21.9%, likely reflecting its central role in core metabolism.

(Notebook: 02_gene_specificity.ipynb)

2. Metal-Specific Genes Are Core-Enriched but Less So Than General Sick Genes

Core fractions across the 22 organisms with pangenome links, reported as both pooled (total core / total genes) and organism-mean (mean of per-organism core fractions):

The pooled metal-specific core fraction (84.8%) is lower than the organism-mean (88.0%), reflecting the influence of organisms with large gene sets and lower core fractions. Both metrics show the same pattern: metal-specific genes are core-enriched above baseline but less so than general sick genes.

All three categories are significantly core-enriched above baseline. Metal-specific genes are the least core-enriched of the three, consistent with specialized metal resistance mechanisms being slightly more likely to reside in the accessory genome than general stress functions — but the difference is modest.

CMH test: The Cochran-Mantel-Haenszel test comparing metal-specific vs general-sick core enrichment across organisms is statistically significant (p=0.011), confirming that metal-specific genes are less core-enriched than general sick genes across organisms. This is consistent with specialized metal resistance mechanisms being modestly more likely to reside in the accessory genome than general cellular functions.

Essential gene caveat: ~14% of protein-coding genes (~82% core) are putatively essential and absent from fitness data. This biases all categories toward core enrichment — the true baseline core fraction is likely lower than 81%, making the reported deltas conservative.

(Notebook: 03_conservation_analysis.ipynb)

3. Metal-Specific Genes Are Enriched for Metal Resistance Functions

Metal-specific genes are 1.64x more likely to match metal-resistance keywords (efflux, transporter, metal, CDF, siderophore, etc.) than general sick genes (12.2% vs 7.8%, Fisher exact OR=1.64, p=2.4e-8). Conversely, general sick genes show slightly higher enrichment for general stress keywords (DNA repair, cell wall, chaperone, etc.) at 11.5% vs 13.7%. This confirms the specificity classification captures biologically meaningful categories.

(Notebook: 04_functional_enrichment.ipynb)

4. Top Novel Candidate Specificity

Three candidates show strong metal-specificity: UCP030820 (67%, oxidoreductase involved in sulfite reduction, important for 7 metals including Cd and Cr), YebC (58%, transcriptional regulator/translation factor spanning 11 organisms and 6 metals), and DUF1043/YhcB (50%, cell division/envelope coordination protein). These are primarily metal-specific rather than general stress genes.

YebC's metal-specificity is mechanistically intriguing. Ignatov et al. (2025) showed YebC functions as a translation factor for proline-rich proteins, resolving ribosome stalling at polyproline motifs. Many metal homeostasis proteins — including P-type ATPases (CopA, ZntA), CDF transporters (CzcD), and metal-binding chaperones — contain proline-rich regions in their cytoplasmic loops. A plausible hypothesis is that YebC is specifically required under metal stress because metal-induced demand for these proline-rich metal transporters creates a translation bottleneck that YebC resolves. Under non-metal conditions, these transporters are not highly expressed and YebC is dispensable — explaining its metal-specific fitness profile.

YfdZ and the Mla/Yrb system (YrbC/D/E) are more pleiotropic — sick under many non-metal conditions. YfdZ's high sick rate (0.268) reflects its known role in alanine biosynthesis. The Mla system's pleiotropic fitness defects are consistent with its established function in maintaining outer membrane integrity under diverse stresses.

DUF39 shows 0% metal-specificity (sick rate 0.637) — it is important for many conditions, not just metals. Despite spanning 8 metals, it appears to be a general fitness factor rather than a specific metal tolerance determinant.

(Notebook: 04_functional_enrichment.ipynb)

5. Novel Candidates Are Not Disproportionately Metal-Specific

Across all 149 novel metal candidate families, 45.6% have a dominant specificity of "metal-specific" — compared to 58.2% for annotated families (Fisher exact OR=0.60, p=0.003). Novel candidates are less metal-specific than annotated ones. This reflects the composition of the novel set: many novel candidates were identified in deeply-profiled organisms (DvH, Btheta, psRCH2) where the high experiment count provides more opportunities to detect pleiotropic effects, pushing genes toward "general sick."

(Notebook: 04_functional_enrichment.ipynb)

6. ICA Module Analysis: Inconclusive

The module-level specificity analysis using z-scored activity profiles found 0 metal-specific modules. The per-module z-normalization produces max |z| values < 2.0 for most metal experiments because metal experiments are a small fraction of total experiments per organism. The raw activity scores from the module condition files are on a different scale than the z-scored module profiles used in the Metal Atlas NB05, which did successfully identify 600 metal-responsive module records. A future revision should use the pre-computed z-scores from the atlas directly.

(Notebook: 04_functional_enrichment.ipynb)

7. Cross-Validation Against Counter Ion Effects

The counter_ion_effects project found 39.8% overlap between metal-important and NaCl-stress genes. This analysis finds 14.7% of metal-important genes are sick under osmotic stress — a 2.7x discrepancy. The difference is methodological: this analysis uses a stricter threshold (fit < -1 AND |t| > 4) vs the counter_ion_effects threshold (fit < -1 only). Additionally, the organism sets differ partially. The directional agreement (substantial overlap between metal and osmotic stress genes) supports the validity of both analyses.

(Notebook: 02_gene_specificity.ipynb)