Involved in the biosynthesis of ambiguines, a family of hapalindole-type alkaloids (PubMed:24180436). Responsible for the synthesis of the isonitrile group on tryptophan using ribulose 5-phosphate as the source of the carbon atom (PubMed:24180436, PubMed:28212039). D-ribulose 5-phosphate + L-tryptophan = (2S)-3-(1H-indol-3-yl)-2-isocyanopropanoate + formaldehyde + H(+) + H2O + hydroxyacetone + phosphate AmbI1 and AmbI3 alone are sufficient to generate Z-3-(2-isocyanoethen)-indole from L-tryptophan and ribulose 5-phosphate, but AmbI2 and AmbI3 are not, suggesting that AmbI2 is functionally redundant in vitro. Belongs to the isocyanide synthase family.