Polyketide-derived polyene systems easily undergo light-induced photoisomerization. The origin of the putative biogenetic precursor of the immunosuppressant SNF4435C/D (3a/b), an isomer of spectinabilin (1), remained unclear. We reinvestigated the metabolic profile of Streptomyces orinoci, a producer of 1, under various growth conditions. Using synthetic 3a as a reference, we found that 3a/b are formed only when the culture is exposed to daylight, which was confirmed in vitro by exposing pure 1 isolated from S. orinoci to daylight and artificial light. We isolated a new nitroaryl-substituted metabolite orinocin (4) from S. orinoci, whose structure was fully established by 1D- and 2D-NMR spectroscopy (including NOESY experiments) and confirmed by total synthesis, proving it to be a truncated homologue of 1 and 2. LC- and GC-MS analyses revealed that 4 originates from 3a by a [2+2] cycloreversion with concomitant formation of mesitylene, which is composed of three propionate units and represents a polyketide not previously described as a natural product. In summary, the formation of 3a/b from 1 is initiated by photoinduced isomerization followed by an 8p–6p electrocyclic rearrangement cascade. The production of 4 from 1 via 3 involves excision of three propionate units, a process termed "polyene splicing", which is unprecedented for natural products and represents an important example of generating structural diversity through nonenzymatic downstream processes. Our results also provide a biosynthetic route to mesitylene as a natural polyketide.