C. Dai, K. Sato
Motivation: The 5-prime untranslated region (UTR) and the start-codon-proximal region of the coding sequence (CDS) jointly influence translation efficiency and local RNA secondary-structure stability, while synonymous codon choices throughout the CDS shape codon adaptation. Because the encoded protein is often predetermined, practical mRNA design must coordinate these quantitative objectives while preserving specified nucleotide sequences and amino-acid identities. Existing generative approaches typically address continuous-valued targeting, explicit sequence constraints, and codon-usage control separately rather than integrating all three within a single model. Results: We present UTR-Diffusion, a diffusion-based framework for 5-prime UTR and 5-prime UTR-CDS junction design. UTR-Diffusion conditions generation on continuous-valued MRL and MFE targets and supports nucleotide-level constraints, amino-acid-level constraints with synonymous-codon flexibility, and codon-adaptiveness control that modulates the sequence-level codon adaptation index (CAI). Systematic evaluations across dense MRL-MFE target grids showed that generated distributions shifted consistently with both targets, retained substantial diversity, and strictly preserved specified nucleotide sequences and amino-acid identities. Codon-adaptiveness control yielded distinct, monotonically ordered CAI levels that closely followed the specified adaptiveness targets. In comparative benchmarks, UTR-Diffusion outperformed representative existing methods in high-MRL optimization and precise MRL targeting for 5-prime UTR design, and achieved higher MRL, less-negative junction MFE, and higher CAI than peptide-preserving baselines in 50-nt 5-prime UTR-CDS junction design.