Expression studies in tetrapyrrole biosynthesis: inverse maxima of magnesium chelatase and ferrochelatase activity during cyclic photoperiods
The synthesis of tetrapyrroles is regulated in anticipation of rhythmic changes in environmental conditions such as light intensity and temperature. To assess the control of the rate-limiting steps of the metabolic flow as well as the distribution of precursors for chlorophyll and heme synthesis, RNA steady-state levels and activities of enzymes involved in tetrapyrrole biosynthesis were analysed from 4-week-old tobacco (Nicotiana tobacum L.) plants grown under photoperiodically changing conditions. The kinetics of RNA levels and the enzyme activities were compared with those from plants which grew subsequent to the light/dark cycles for 48 h under constant light or dark conditions. The analysis revealed that the two peak activities for 5-aminolevulinic acid synthesis and of magnesium-protoporphyrin IX chelatase (Mg-chelatase) corresponded with the highest accumulation of the transcripts encoding glutamyl-tRNA reductase and CHL H, a subunit of Mg-chelatase, in the first half of the light period during a light/dark cycle. The activity of ferrochelatase (Fe-chelatase) and the level of its RNA showed a maximum just at the transition from light to dark and oscillated with a phase approximately opposite to that of Mg-chelatase activity. The control of 5-aminolevulinic acid synthesis and of the allocation of protoporphyrin IX to Mg- or Fe-chelatase probably reflect the functional coordination of tetrapyrrole biosynthesis in response to daily fluctuations in tetrapyrrole requirements. It is suggested that the coordination of expression and enzyme activities allows, in the light phase, an extensive flow of substrates into the chlorophyll-synthesizing branch of the metabolic pathway and, after the transition from light to dark, a channeling into the heme biosynthetic pathway. Implications for feedback control in the pathway are discussed.
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