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 that 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 the nitrogen source. The initial kinetics of RNA accumulation allowed an ordering of the genes with respect to the timing of their expression, with individual genes being expressed very early, early, intermediately, or late after induction. For very early genes, significantly increased transcript levels were detected within 30 min. This fast response suggests that gene expression is rapidly activated after removal of the nitrogen source. The accumulation of transcripts from the very early, early, and intermediate genes preceded the appearance of mating competence. Though transcript levels of several very early genes fluctuated during subsequent incubation in nitrogen-free 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.