Involved in the biosynthesis of rhabduscin, a tyrosine derivative which is a potent inhibitor of phenoloxidase, a key component of the insect's innate immune system (PubMed:22711807). Responsible for the synthesis of the isonitrile group on tyrosine using the C2 of ribulose 5-phosphate as the source of the carbon atom (Probable). D-ribulose 5-phosphate + L-tyrosine = (2S)-3-(4-hydroxyphenyl)-2-isocyanopropanoate + formaldehyde + H(+) + H2O + hydroxyacetone + phosphate Deletion of isnA and isnB abolishes rhabduscin biosynthesis and decreases virulence of the strain. Belongs to the isocyanide synthase family.