Tobacco Mg protoporphyrin IX methyltransferase is involved in inverse activation of Mg porphyrin and protoheme synthesis

Abstract

SummaryProtoporphyrin, a metabolic intermediate of tetrapyrrole biosynthesis, is metabolized by Mg chelatase and ferrochelatase and is directed into the Mg-branch for chlorophyll synthesis and in the Fe-branch for protoheme synthesis respectively. Regulation of the enzyme activities at the beginning of this branchpoint ensures accurate partition of protoporphyrin, but is still not entirely understood. Transgenic tobacco plants were generated that express antisense or sense RNA for inhibited and excessive expression of Mg protoporphyrin methyltransferase (MgPMT) respectively. This enzyme accepts Mg protoporphyrin from Mg chelatase and catalyses the transfer of a methyl group to the carboxyl group of the C13-propionate side chain. Low MgPMT activity is correlated with reduced Mg chelatase activity and a low synthesis rate of 5-aminolevulinate, but with enhanced ferrochelatase activity. In contrast, high MgPMT activity leads to inverse activity profiles: high activities of Mg chelatase and for 5-aminolevulinate synthesis, but reduced activity of ferrochelatase, indicating a direct influence of MgPMT in combination with Mg chelatase on the metabolic flux of ALA and the distribution of protoporphyrin into the branched pathway. The modified enzyme activities in tetrapyrrole biosynthesis in the transgenic plants can be explained with changes of certain corresponding mRNA contents: increased 5-aminolevulinate synthesis and Mg chelatase activity correlate with enhanced transcript levels of the HemA, Gsa, and CHLH gene encoding glutamyl-tRNA reductase, glutamate-1-semialdehyde aminotransferase and a Mg chelatase subunit respectively. It is proposed that reduced and increased MgPMT activity in chloroplasts is communicated to the cytoplasm for modulating transcriptional activities of regulatory enzymes of the pathway.

ArabidopsisSAMT1 Defines a Plastid Transporter Regulating Plastid Biogenesis and Plant Development

“…A possible implication of this result is that the SAM-dependent conversion of MgProto IX to MgProto IX Me becomes severely disrupted when the import of SAM into plastids is restricted by SAMT1 activity. Interestingly, tobacco plants expressing antisense SAM-MgProto IX methyltransferase exhibited chlorotic leaves and a reduced-growth phenotype (Alawady and Grimm, 2005), as seen in Nb SAMT1-silenced plants (Figures 7A and 7C). The kinetic constants of enzymes catalyzing the different steps of subplastid prenyllipid biosynthetic pathways are not yet available.…”

Genomics-based dissection of the cross-talk of chloroplasts with the nucleus and mitochondria in Arabidopsis

“…Experimental evidence for a strict correlation between enzyme activities in the ''Mg branch'' of tetrapyrrole biosynthesis and gene expression of certain nuclear genes for photosynthesis or tetrapyrrole formation (Papenbrock et al, 2000;Yaronskaya et al, 2003;Wilde et al, 2004;Alawady and Grimm, 2004) substantiated the gun mutant analysis, prompting proposals in favour of a role of Mg porphyrins and/or enzymes of this biosynthetic pathway as sensors, or as mediating a plastid signal. GUN5 was originally suggested to act as a sensor (Mochizuki et al, 2001), but the idea was recently revised in favour of the tetrapyrrole intermediate Mg-protoporphyrin IX, a proposed signalling molecule between chloroplast and nucleus (Strand et al, 2003; Fig.…”

Recessiveness and Dominance in Barley Mutants Deficient in Mg-Chelatase Subunit D, an AAA Protein Involved in Chlorophyll Biosynthesis

“…Mutants unable to synthesize Mg-protoporphyrin contain only unfenestrated thylakoids and lack prolamellar bodies (von Wettstein et al, 1995). Interestingly, Mg-protoporphyrin suppresses nuclear expression of Lhab1 and other genes encoding chloroplast-localized proteins (Kropat et al, 2000;Strand et al, 2003;Alawady and Grimm, 2005;Gadjieva et al, 2005), which suggests that Mg-protoporphyrin plays the essential role of signal molecule in the regulation of these processes. In addition, Mg-protoporphyrin and heme are known to regulate the first biosynthetic steps of their common pathway (Vasileuskaya et al, 2005).…”