Low-expression genes induced by nitrogen starvation and subsequent sexual differentiation in Chlamydomonas reinhardtii, isolated by the differential display technique

Genes that are expressed upon a shift to nitrogen-free medium, an event tha t initiates gametogenesis, were identified in Chlamydomonas reinhardtii by using the differential display technique. Ten different cDNAs were isolated and shown to have increased levels of their transcripts upon removal of th e nitrogen source. The initial kinetics of RNA accumulation allowed an orde ring of the genes with respect to the timing of their expression, with indi vidual genes being expressed very early, early, intermediately, or late aft er induction. For very early genes, significantly increased transcript leve ls were detected within 30 min. This fast response suggests that gene expre ssion is rapidly activated after removal of the nitrogen source. The accumu lation of transcripts from the very early, early, and intermediate genes pr eceded the appearance of mating competence. Though transcript levels of sev eral very early genes fluctuated during subsequent incubation in nitrogen-f ree medium, most of them exhibited maxima when the highest level of mating competence was attained. One of these very early genes was shown to encode a urate oxidase type-II enzyme.