Carotenoids are indispensable natural pigments to plants and humans. Phytoene synthase (PSY), the rate-limiting enzyme in the carotenoid biosynthetic pathway, and ORANGE (OR), a regulator of chromoplast differentiation and enhancer of carotenoid biosynthesis, represent two key proteins that control carotenoid biosynthesis and accumulation in plants. However, little is known about the mechanisms underlying their posttranscriptional regulation. Here we report that PSY and OR family proteins [Arabidopsis thaliana OR (AtOR) and AtOR-like] physically interacted with each other in plastids. We found that alteration of OR expression in Arabidopsis exerted minimal effect on PSY transcript abundance. However, overexpression of AtOR significantly increased the amount of enzymatically active PSY, whereas an ator ator-like double mutant exhibited a dramatically reduced PSY level. The results indicate that the OR proteins serve as the major posttranscriptional regulators of PSY. The ator or ator-like single mutant had little effect on PSY protein levels, which involves a compensatory mechanism and suggests partial functional redundancy. In addition, modification of PSY expression resulted in altered AtOR protein levels, corroborating a mutual regulation of PSY and OR. Carotenoid content showed a correlated change with OR-mediated PSY level, demonstrating the function of OR in controlling carotenoid biosynthesis by regulating PSY. Our findings reveal a novel mechanism by which carotenoid biosynthesis is controlled via posttranscriptional regulation of PSY in plants.arotenoids are a group of C40 isoprenoids synthesized in chloroplasts, chromoplasts, and other plastids in plants. Carotenoids serve as components of photosynthetic machinery, precursors for phytohormones, and important contributors to fruit nutritional quality and flower color (1, 2). The carotenoid biosynthetic pathway in higher plants has been well defined. However, identification of the regulatory mechanisms underlying carotenoid biosynthesis remains a challenge.Phytoene synthase (PSY) catalyzes the first committed step in carotenoid biosynthesis and controls carbon flux into the carotenoid biosynthetic pathway (1-5). Alteration of PSY expression exerts profound effects on carotenoid content (6-11). A number of factors are known to affect PSY gene expression (12)(13)(14)(15)(16)(17)(18). PSY is found to be repressed by phytochrome-interacting factors in etiolated Arabidopsis seedlings (16). PSY1 expression in tomato fruits is reported to be regulated by cis-carotenoids (14) and requires the MADS-Box transcription factor RIPENING INHIBITOR (18). Recently, it was discovered that PSY protein levels in carrot roots are modulated by a negative feedback emerging from carotenoids (19). The crucial role of PSY in carotenogenesis and the multiple factors affecting its expression suggest a complex regulatory system involved in controlling PSY. However, the factors involved in posttranscriptional regulation of PSY within plastids remain a mystery. No proteins have been re...

To fully understand how bacteria respond to their environment, it is essential to assess genome-wide transcriptional activity. New high-throughput sequencing technologies make it possible to query the transcriptome of an organism in an efficient unbiased manner. We applied a strand-specific method to sequence bacterial transcripts using Illumina's high-throughput sequencing technology. The resulting sequences were used to construct genome-wide transcriptional profiles. Novel bioinformatics analyses were developed and used in combination with proteomics data for the qualitative classification of transcriptional activity in defined regions. As expected, most transcriptional activity was consistent with predictions from the genome annotation. Importantly, we identified and confirmed transcriptional activity in areas of the genome inconsistent with the annotation and in unannotated regions. Further analyses revealed potential RpoN-dependent promoter sequences upstream of several noncoding RNAs (ncRNAs), suggesting a role for these ncRNAs in RpoNdependent phenotypes. We were also able to validate a number of transcriptional start sites, many of which were consistent with predicted promoter motifs. Overall, our approach provides an efficient way to survey global transcriptional activity in bacteria and enables rapid discovery of specific areas in the genome that merit further investigation.

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