Phosphomevalonate kinase regulates the MVA/MEP pathway in mango during ripening.

Mango is a popular tropical fruit with a great diversity in taste and aroma, contributed primarily by terpenoids. Phosphomevalonate kinase (PMK) is a key enzyme for isoprenoid biosynthesis in the mevalonic acid (MVA) pathway responsible for terpenoids. In this study, two cultivars of mango, "Dashehari" and "Banganpalli", showing opposite spatio-temporal patterns of ripening polarity, were investigated for studying the role of MiPMK in aroma production. MiPMK transcription and enzyme activity increased during ripening in both varieties. Expression in the early-ripening inner zones preceded that in the later-ripening outer zones of "Dashehari" while it was higher in the early ripening outer zones in "Banganpalli". Polypeptide sequences of the two enzymes showed differences in a few amino acids that were also reflected in kinetic properties such as specific activity and pH optima. Silencing of MiPMK in "Dashehari" fruit by VIGS suppressed the kinase activity and led to changes in relative contributions of the mevalonic acid (MVA) and methylerythritol 4-phosphate (MEP) pathways. This also altered the fruit metabolite profile with a reduction/disappearance of sesquiterpenes such as geranyl geraniol, trans-farnesol, ß-caryophyllene, ß-pinene, bisabolene and guaiane but the appearance of menthol and d-limonene in silenced fruit. The study shows that MiPMK levels may control downstream metabolite flux of the MVA pathway in mango.

The nucleus-encoded HCF107 gene of Arabidopsis provides a link between intercistronic RNA processing and the accumulation of translation-competent psbH transcripts in chloroplasts.

To understand the functional significance of RNA processing for the expression of plastome-encoded photosynthesis genes, we investigated the nuclear mutation hcf107 of Arabidopsis. The mutation is represented by two alleles, both of which lead to a defective photosystem II (PSII). In vivo protein labeling, in vitro phosphorylation, and immunoblot experiments revealed that the psbB gene product (CP47) and an 8-kD phosphoprotein, the psbH gene product (PsbH), are absent in mutant plants. PsbH and PsbB are essential requirements for PSII assembly in photosynthetic eukaryotes, and their absence in hcf107 is consistent with the PSII-less mutant phenotype. RNA gel blot hybridizations showed that the hcf107 mutation specifically impairs the accumulation of some but not all oligocistronic psbH transcripts that are released from the pentacistronic psbB-psbT-psbH-petB-petD precursor RNA by intergenic endonucleolytic cleavage. In contrast, neither the levels nor the sizes of psbB-containing RNAs are affected. S1 nuclease protection analyses revealed that psbH RNAs are lacking only where psbH is the leading cistron and that they are processed at position -45 in the 5' leader segment of psbH. These data and additional experiments with the cytochrome b(6)f complex mutant hcf152, which is defective in 3' psbH processing, suggest that only those psbH-containing transcripts that are processed at their -45 5' ends can be translated. Secondary structure analysis of the 5' psbH leader predicted the formation of stable stem loops in the nonprocessed transcripts, which are unfolded by processing at the -45 site. We propose that this unfolding of the psbH leader segment as a result of RNA processing is essential for the translation of the psbH reading frame. We suggest further that HCF107 has dual functions: it is involved in intercistronic processing of the psbH 5' untranslated region or the stabilization of 5' processed psbH RNAs, and concomitantly, it is required for the synthesis of CP47.