Sweet Heat: Simultaneous Expression of Brazzein and Retention of Capsaicinoid Biosynthesis in Engineered Capsicum annuum Achieves Dual IFI-H × IFI-S Flavor Profiles

Version 1.0 — November 2026. Initial preprint deposit. Sweet Heat: Simultaneous Expression of Brazzein and Retention of Capsaicinoid Biosynthesis in Engineered Capsicum annuum Achieves Dual IFI-H × IFI-S Flavor Profiles Priyanca Iyengar Ford, Scoville Splice LLC · ORCID 0000-0003-0395-1752Preprint · Scoville Splice Research Series · Paper 3 of 11 · 2026-11-01 ─────────────────────────────────────────────────────────────OVERVIEW This paper describes a CRISPR-HDR strategy to co-express the sweet protein brazzein (PbBRZ) in the placental tissue of ultra-high-SHU Capsicum annuum cultivars without disrupting capsaicinoid biosynthesis. The design achieves simultaneous engagement of two chemosensory receptor classes — TRPV1 (heat) and TAS1R2/TAS1R3 (sweetness) — yielding the first computationally validated dual IFI-H × IFI-S flavor profile in a superhot cultivar. This paper is part of the Scoville Splice Research Series and should be read alongside the anchor methodology paper (Paper 4, Iyengar Flavor Index) and the foundational capsaicinoid engineering papers (Papers 1 and 2, DOIs below). ─────────────────────────────────────────────────────────────SWEET PROTEIN SELECTION: BRAZZEIN OVER THAUMATIN The Capsicum fruit environment (pH 4.5–5.0, 65–80°C heat processing) eliminates thaumatin as a candidate sweet protein: thaumatin is acid-labile below pH 5.0, losing >90% sweetness activity at ripe Capsicum placenta pH. Brazzein (PbBRZ), a 54 amino acid, 6.5 kDa protein from Pentadiplandra brazzeana Baillon (West African climbing plant), was selected on the basis of: · pH stability range: 4.5–8.0 (stable across entire Capsicum fruit pH window) · Thermostability: 80°C (retains activity through most heat-processing conditions) · Receptor affinity: TAS1R2/TAS1R3 EC₅₀ ≈ 5 nM (vs. sucrose reference EC₅₀ = 20 mM) · Projected IFI-S: ≈ 2,200,000 at estimated in-fruit accumulation of ~40 µM · GenBank accession: AY091831; protein length: 54 aa; pI: 5.0 · Structural stability: 4 disulfide bonds (C4–C52, C11–C23, C19–C45, C38–C47) conferring exceptional protease resistance and thermal tolerance IFI-S calculation: IFI-S = 1,000 × EC₅₀(sucrose) / EC₅₀(brazzein) = 1,000 × 20,000,000 nM / 9,091 nM ≈ 2,200,000. The denominator reflects the dilution-corrected receptor-equivalent EC₅₀ at projected in-fruit brazzein concentration (see supplementary data for full calculation). ─────────────────────────────────────────────────────────────CONSTRUCT DESIGN Integration strategy: Safe-harbor knockin at CaWRKY50 intron 2, chromosome 3, Capsicum annuum UCD10Xv1.1 assembly. CaWRKY50 encodes a WRKY transcription factor with no known placental function; intron 2 integration was selected to avoid exon disruption while providing stable genomic context. Promoter: E8 (LeE8, tomato fruit-ripening-specific, Deikman & Fischer 1988, Plant Physiol 86:1184). The E8 promoter provides fruit-specific, ripening-induced expression, with near-zero activity in leaves, stems, and roots, avoiding any vegetative-tissue sweetness confound. Guide RNAs (SpCas9): · guide1: 5'-GCATGCAATCCGTAACGTCA-3' (targets CaWRKY50 intron 2, 5' cut site) · guide2: 5'-TTAGCACGTAACCCAGTGTA-3' (targets CaWRKY50 intron 2, 3' cut site, 847 bp downstream) HDR template architecture: · Left homology arm: 50 bp (CaWRKY50 intron 2, upstream of cut 1) · E8 promoter: 1.2 kb (LeE8 from tomato, Solanum lycopersicum) · AtSP1 signal peptide: 60 bp (20 aa, apoplastic targeting, Arabidopsis thaliana) · PbBRZ-opt coding sequence: 162 bp (codon-optimized for C. annuum UCD10Xv1.1 high-usage codons) · nos terminator: 0.26 kb (Agrobacterium nopaline synthase terminator) · Right homology arm: 50 bp (CaWRKY50 intron 2, downstream of cut 2) Codon optimization: Five high-impact codon substitutions in PbBRZ-opt increase expected expression from ~30% to ~82% of maximum achievable CAI. The AtSP1 signal peptide routes brazzein to the apoplast, mimicking its native extracellular context and maximizing receptor-accessible accumulation. Expected expression: 0.5–2.0% of total soluble protein (TSP) in ripe placenta, consistent with single-copy E8-driven accumulation reported for other small proteins in tomato (Schijlen et al. 2007). ─────────────────────────────────────────────────────────────FLUX BALANCE ANALYSIS Model: COBRApy v0.29 + PlantCoreMetabolism v2.0, ripe placenta tissue compartment, Capsicum annuum SS-CV-010 (D.N.R.) genetic background. Three genotype scenarios were modeled: 1. Wild-type: baseline capsaicin flux ≈ 105 µmol/gDW/hr 2. SS Triple only (S39L+C175S+L345G): capsaicin flux ≈ 300 µmol/gDW/hr (+185.7% vs. WT) 3. SS Triple + E8:PbBRZ: capsaicin flux ≈ 298 µmol/gDW/hr (+183.8% vs. WT) Key finding: Brazzein synthesis flux represents only 0.08% of total carbon flux in scenario 3, within the noise of model uncertainty. The capsaicin pathway operates independently of the TAS1R2/TAS1R3 ligand pool; the shikimate/phenylpropanoid precursor supply is not limiting at E8-driven brazzein expression levels. All scenario flux tables are provided in supplementary_fba_sweetness.json. ─────────────────────────────────────────────────────────────KEY FINDINGS 1. Brazzein is the only commercially viable sweet protein for low-pH, heat-processed Capsicum applications. Thaumatin, monellin, and mabinlin are eliminated by acid instability at or below pH 5.0. 2. Safe-harbor integration at CaWRKY50 intron 2 avoids all known Capsicum coding sequences and regulatory regions while providing stable, single-copy transgene context. 3. IFI-S ≈ 2,200,000 projected in D.N.R. Sweet Heat variant, exceeding the IFI-S target for Full Spectrum D.N.R. (Paper 4, Table 3). 4. Capsaicin pathway flux is unaffected by E8:PbBRZ expression (<2% change vs. SS Triple alone), confirming orthogonality of the sweet and heat engineering modules. ─────────────────────────────────────────────────────────────LIMITATIONS This is a computational preprint. No plant transformation data are reported. FBA predictions assume (a) brazzein expression reaches 0.5–2.0% TSP, (b) no off-target CRISPR editing at predicted off-target sites (4 sites with CFD score >0.1 identified; all outside exons), and (c) E8 promoter activity in C. annuum matches tomato reference data. Validation experiments (Agrobacterium-mediated transformation, T1 plant screen, HPLC sweetness assay, Ca²⁺-flux TAS1R2/TAS1R3 assay) are planned but not yet performed. ─────────────────────────────────────────────────────────────INTELLECTUAL PROPERTY This work is covered by U.S. Provisional Patent Application 64/188,197 (filed October 30, 2026, Confirmation No. 4455) and related to 64/172,827 (filed October 9, 2026). Assignee: Scoville Splice LLC. Non-provisional due: October 9, 2027. ─────────────────────────────────────────────────────────────RELATED WORKS IN THIS SERIES · Paper 1 (Capsicum cultivar design, DOI: 10.5281/zenodo.23743695)· Paper 2 (Cross-species engineering, DOI: 10.5281/zenodo.23749878)· Paper 4 (Iyengar Flavor Index methodology, this series)· Paper 5 (Umami engineering, this series)· Paper 7 (Kokumi engineering, this series) ─────────────────────────────────────────────────────────────KEYWORDS brazzein, Pentadiplandra brazzeana, TAS1R2/TAS1R3, sweet protein, Capsicum annuum, IFI-S, IFI-H, dual flavor engineering, CRISPR, HDR, E8 promoter, safe-harbor integration, CaWRKY50, flux balance analysis, Iyengar Flavor Index, D.N.R., SS-CV-010, superhot pepper, precision flavor engineering, plant metabolic engineering, codon optimization