Step 1 β Normalize each omics layer independently. RNA-seq: QC with fastp/fastp-like tools, align (HISAT2/STAR), quantify (RSEM/featureCounts), normalize with DESeq2 VST or limma-voom. Proteomics: use MaxQuant (LFQ) or Spectronaut (DIA) with 1% FDR, normalize with VSN/quantile/Loess via NormalyzerDE, then limma for differential abundance .
Step 2 β Map identifiers to a shared feature space. Convert gene IDs, protein IDs, and UniProt accessions to a common ortholog/ensembl key; retain only features quantified in both datasets. Account for protein isoforms (many-to-one mapping) and housekeeping-gene overlap.
Step 3 β Compute correlations. Calculate Pearson and Spearman correlations on log2-transformed, matched-sample abundances. Report per-condition and per-gene correlations. Publish reports show typical ranges of r = 0.46β0.72 across bacterial species/stresses, with osmotic stress producing the strongest decoupling .
Step 4 β Nine-quadrant and hub analysis. Plot log2FC(protein) vs log2FC(mRNA); classify genes into concordant up/down, protein-only, mRNA-only, and discordant quadrants . Build STRING/Cytoscape PPI networks from concordant and hub genes (e.g., CytoHubba).
Step 5 β Interpret discordance. Low correlation indicates post-transcriptional or post-translational regulation β not failure. Validate key discordant genes by qRT-PCR and targeted proteomics; consider ribosome profiling or polysome assays to test translation-level control .
Known pitfalls: incomplete proteome coverage (32β48% of CDSs in bacteria) biases correlation; mismatched timepoints confound; correlative findings require functional validation for causal inference .
Reported benchmark ranges (not simulated) from a 3-species, 11-condition stress study .
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