Two julichrome monomers, julichromes Q 11 (1) and Q 12 (2), along with five known julichromes (Q 10 , Q 3 · 5 , Q 3 · 3 , Q 6 · 6 , Q 6 , 3 -7) and four known anthraquinones (chrysophanol, 4-acetylchrysophanol, islandicin, huanglongmycin A, 8-11), were isolated from the marine gastropod mollusk Batillaria zonalis-associated Streptomyces sampsonii SCSIO 054. This is the first report of julichromes isolated from a marine source. Extensive dissection of 1D and 2D NMR datasets combined with X-ray crystallography enabled rigorous elucidation of the previously reported configurations of julichrome Q 3 · 5 (4) and related julichrome Q 3 · 3 (5); both of the configuration at C(9) needs to be revised. In addition, julichrome Q 12 (2) was found to display antibacterial activity against Micrococcus luteus and Bacillus subtilis with MICs of 2.0 and 8.0 μg mL À 1 ; four compounds (1, 3, 6, 7) also showed inhibitory activities against an array of methicillin-resistant Staphylococcus aureus, S. aureus and S. simulans AKA1 with MIC values ranging from 8 to 64 μg mL À 1 . Figure 4. Single-crystal X-ray structure of 4 (ORTEP drawing, ellipsoid probability 30 %).

The biosynthetic gene cluster governing the production of antibacterial julichromes was identified from marine gastropod mollusk-associated Streptomyces sampsonii SCSIO 054. Post-PKS assembly/tailoring enzymes JuiL, JuiM, JuiI, and JuiN represent key assembly enzymes. JuiL serves as a ketoreductase. JuiM is an acetyltransferase. JuiI carries out an intriguing biaryl coupling of two julichrome Q 6 units (immediate JuiL, JuiM product) to afford julichrome Q 6−6 . JuiN carries out tailoring steps on julichrome Q 6−6 , transforming Q 6−6 into Q 3−3 .