N-Methyl Mesoporphyrin IX Inhibits Phycocyanin, but Not Chlorophyll Synthesis in Cyanidium caldarium

Abstract

The ability of N-methyl mesoporphyrin IX (NMMP) to block heme synthesis by specificaly inhibiting enzymic iron insertion into protoporphyrin IX was exploited to test whether heme is a precursor of the biHn chromophore of phycocyanin (PC). A strain of the unicellular rhodophyte Cyanidiwn cadbiwm which forms normal amounts of both chorophyll (Chi) Biliproteins are the major light-harvesting pigments in bluegreen, red, and cryptomonad algae. The biliproteins are composed of open-chain phycobilin chromophore molecules which are covalently bound to proteins. Although the pigments are present in high abundance in the organisms in which they occur, their biosynthesis is poorly understood. Structural considerations suggest that phycobilins arise from macrocycle ring-opening of a porphyrin or metalloporphyrin precursor, but the relevant enzymic steps have not been observed in vitro (Fig. 1).Recently, Brown et al. (5) reported that exogenous '4C-heme was incorporated into phycocyanobilin, the bilin chromophore of PC2, but not into Chl, in greening cultures of the unicellular rhodophyte Cyanidium caldarium. This result suggests that heme is a metabolic precursor to the phycobilins. Others have proposed that the phycobilins might arise from the Mg-porphyrin branch of the tetrapyrrole pathway leading to Chl (1, 9). Possible biosynthetic paths to phycobilins are illustrated in Figure 2.

Biosynthesis of Protoheme and Heme a Precursors Solely from Glutamate in the Unicellular Red Alga Cyanidium caldarium

“…Wild-type cells of C. caldarium and a nonpigmented mutant (GGB-Y), incapable of forming Chl or phycobilins, were a generous gift from Dr. R. F. Troxler (Boston University School of Medicine). The cells were grown axenically in the dark at 42°C on a glucose-based heterotrophic medium as described previously (3). Each culture consisted of 60 ml of cell suspension in a 125-ml flask.…”

Further Characterization of the Magnesium Chelatase in Isolated Developing Cucumber Chloroplasts

“…5) which, up to this point, have been considered to be specific ferrochelatase inhibitors (10,11). Although these compounds have been reported to have no effect on Chl synthesis (and presumably Mg-chelatase) in vivo (2,6,19) (10).…”

Incorporation of 1‐deoxy‐d‐xylulose into isoprene and phytol by higher plants and algae

“…The mineral medium of Beale et al [18] was applied with modifications: (NFL^SC^ 2 mM, NH4CI 6 mM, KH2PO4 5 mM, Mg SO4 2.5 mM, CaCl 2 0.1 mM, pH 4.5. After 14 days the cells (1 g dry weight) were harvested by centrifugation.…”