The trpA gene on the plastid genome of Cyanidium caldarium strain RK-1.

The trpA gene (for the alpha subunit of tryptophan synthase) was found on the plastid genome of the "primitive" unicellular red alga Cyanidium caldarium strain RK-1. This is the first example of an actively-transcribed gene for tryptophan synthase encoded on a plastid genome. In contrast to trpA, trpB (the gene for the beta subunit of tryptophan synthase) was encoded in the cell nucleus. Considering the primitive characteristics of C. caldarium, trpB must have been lost from the plastid genome before trpA.