BLUE-LIGHT-REGULATED EXPRESSION OF GENES FOR 2 EARLY STEPS OF CHLOROPHYLL BIOSYNTHESIS IN CHLAMYDOMONAS-REINHARDTII

In light:dark-synchronized cultures of Chlamydomonas reinhardtii, the genes encoding the enzymes for two early steps of chlorophyll biosynth esis, glutamate-1-semialdehyde aminotransferase (gsa) and delta-aminol evulinic acid dehydratase (alad), are expressed at high levels early i n the light phase, just prior to a rapid burst of chlorophyll synthesi s. Induction of gsa mRNA in synchronized cells is totally dependent on light, whereas induction of alad mRNA occurs to approximately one-hal f the light-induced level even in cells kept in the dark during the li ght phase and appears to be dependent on the cell cycle or a circadian rhythm. gsa mRNA and alad mRNA accumulation is induced by light that was passed through blue (400-480 nm) or green (490-590 nm) filters but not by light that was passed through orange (>560 nm) or red (>610 nm ) filters, indicating the participation of a blue-light photoreceptor system rather than a protochlorophyllide- or rhodopsin-based photorece ptor. Light induction of gsa mRNA accumulation is absent in a caroteno id-deficient mutant, which suggests that a carotenoid-containing blue- light photoreceptor is involved. In contrast, pretreatment of wild-typ e cells with either of two flavin antagonists, phenylacetic acid and K I, does not prevent the light induction. In the later part of the ligh t phase, the gsa mRNA level decreases more rapidly than that of alad m RNA. Turnover studies indicate that the half-life of alad mRNA is twic e that of gsa mRNA. This difference in mRNA stability partially accoun ts for the more rapid decline in gsa mRNA levels after the peak of lig ht induction is reached. Thus, differential blue-light induction and s tability of mRNAs regulates the expression of these two chlorophyll bi osynthetic genes.