KRAS mutations are one of the most common oncogenic drivers in human cancer. While small molecule inhibitors for the G12C mutant have been successfully developed, allele-specific inhibition for other KRAS hotspot mutants remains challenging. Here we report the discovery of covalent chemical ligands for the common oncogenic mutant K-Ras(G12R). These ligands bind in the Switch II pocket and irreversibly react with the mutant arginine residue. An X-ray crystal structure reveals an imidazolium condensation product formed between the α,β-diketoamide ligand and the εand η-nitrogens of arginine 12. Our results show that arginine residues can be selectively targeted with small molecule electrophiles despite their weak nucleophilicity and provide the basis for the development of mutant-specific therapies for K-Ras(G12R)-driven cancer.

Filamentous marine cyanobacteria make a variety of bioactive molecules that are produced by polyketide synthases, non-ribosomal peptide synthetases, and hybrid pathways that are encoded by large biosynthetic gene clusters. These cyanobacterial natural products represent potential drugs leads; however, thorough pharmacological investigations have been impeded by the limited quantity of compound that is typically available from the native organisms.Additionally, investigations of the biosynthetic gene clusters and enzymatic pathways have been difficult due to the inability to conduct genetic manipulations in the native producers. Here we report a set of genetic tools for the heterologous expression of biosynthetic gene clusters in the cyanobacteria Synechococcus elongatus PCC 7942 and Anabaena (Nostoc) PCC 7120. To facilitate the transfer of gene clusters in both strains, we engineered a strain of Anabaena that contains S. elongatus homologous sequences for chromosomal recombination at a neutral site and devised a CRISPR-based strategy to efficiently obtain segregated double recombinant clones of Anabaena. These genetic tools were used to express the large 28.7 kb cryptomaldamide biosynthetic gene cluster from the marine cyanobacterium Moorena (Moorea) producens JHB in both model strains. S. elongatus did not produce cryptomaldamide, however high-titer production of cryptomaldamide was obtained in Anabaena. The methods developed in this study will facilitate the heterologous expression of biosynthetic gene clusters isolated from marine cyanobacteria and complex metagenomic samples.

Chemical investigation on a marine sponge, Dactylospongia elegans, yielded five new γ-oxygenated butenolide sesterterpene derivatives, dactylospenes A–E (1–5), as well as two known biosynthetically related compounds, luffariellolide (6) and furospinosulin B (7). The structures of these compounds were elucidated on the basis of their spectroscopic data, experimental and calculated electronic circular dichroism (ECD) analysis, as well as comparison of the NMR data with those of known analogs. These metabolites are the first γ-oxygenated butenolide sesterterpenes to be reported from this genus. These compounds were evaluated in antimicrobial, anti-inflammatory, and cytotoxic assays. Only compounds 1, 3, and 6 exhibited moderate cytotoxicity against DU145, SW1990, Huh7, and PANC-1 cancer cell lines with IC50 values in the range of 2.11–13.35 μM. Furthermore, compound 2, without cytotoxicity, exhibited significant inhibitory effects (inhibitory rate 77.5%) on nitric oxide production induced by lipopolysaccharide at 10 μM.