The lifestyle of prokaryotic organisms influences the repertoire of promiscuous enzymes

The metabolism of microbial organisms and its diversity are partly the result of an adaptation process to the characteristics of the environments that they inhabit. In this work, we analyze the influence of lifestyle on the content of promiscuous enzymes in 761 nonredundant bacterial and archaeal genomes. Promiscuous enzymes were defined as those proteins whose catalytic activities are defined by two or more different Enzyme Commission (E.C.) numbers. The genomes analyzed were categorized into four lifestyles for their exhaustive comparisons: free‐living, extremophiles, pathogens, and intracellular. From these analyses we found that free‐living organisms have larger genomes and an enrichment of promiscuous enzymes. In contrast, intracellular organisms showed smaller genomes and the lesser proportion of promiscuous enzymes. On the basis of our data, we show that the proportion of promiscuous enzymes in an organism is mainly influenced by the lifestyle, where fluctuating environments promote its emergence. Finally, we evidenced that duplication processes occur preferentially in metabolism of free‐living and extremophiles species. Proteins 2015; 83:1625–1631. © 2015 Wiley Periodicals, Inc.     

Improvement of pea biomass and seed productivity by simultaneous increase of phloem and embryo loading with amino acids

The development of sink organs such as fruits and seeds strongly depends on the amount of nitrogen that is moved within the phloem from photosynthetic‐active source leaves to the reproductive sinks. In many plant species nitrogen is transported as amino acids. In pea (Pisum sativum L.), source to sink partitioning of amino acids requires at least two active transport events mediated by plasma membrane‐localized proteins, and these are: (i) amino acid phloem loading; and (ii) import of amino acids into the seed cotyledons via epidermal transfer cells. As each of these transport steps might potentially be limiting to efficient nitrogen delivery to the pea embryo, we manipulated both simultaneously. Additional copies of the pea amino acid permease PsAAP1 were introduced into the pea genome and expression of the transporter was targeted to the sieve element‐companion cell complexes of the leaf phloem and to the epidermis of the seed cotyledons. The transgenic pea plants showed increased phloem loading and embryo loading of amino acids resulting in improved long distance transport of nitrogen, sink development and seed protein accumulation. Analyses of root and leaf tissues further revealed that genetic manipulation positively affected root nitrogen uptake, as well as primary source and sink metabolism. Overall, the results suggest that amino acid phloem loading exerts regulatory control over pea biomass production and seed yield, and that import of amino acids into the cotyledons limits seed protein levels.     

Lack of fructose 2,6‐bisphosphate compromises photosynthesis and growth in Arabidopsis in fluctuating environments

The balance between carbon assimilation, storage and utilisation during photosynthesis is dependent on partitioning of photoassimilate between starch and sucrose, and varies in response to changes in the environment. However, the extent to which the capacity to modulate carbon partitioning rapidly through short‐term allosteric regulation may contribute to plant performance is unknown. Here we examine the physiological role of fructose 2,6‐bisphosphate (Fru‐2,6‐P₂) during photosynthesis, growth and reproduction in Arabidopsis thaliana (L.). In leaves this signal metabolite contributes to coordination of carbon assimilation and partitioning during photosynthesis by allosterically modulating the activity of cytosolic fructose‐1,6‐bisphosphatase. Three independent T‐DNA insertional mutant lines deficient in 6‐phosphofructo‐2‐kinase/fructose‐2,6‐bisphosphatase (F2KP), the bifunctional enzyme responsible for both the synthesis and degradation of Fru‐2,6‐P₂, lack Fru‐2,6‐P₂. These plants have normal steady‐state rates of photosynthesis, but exhibit increased partitioning of photoassimilate into sucrose and have delayed photosynthetic induction kinetics. The F2KP‐deficient plants grow normally in constant environments, but show reduced growth and seed yields relative to wildtype plants in fluctuating light and/or temperature. We conclude that Fru‐2,6‐P₂ is required for optimum regulation of photosynthetic carbon metabolism under variable growth conditions. These analyses suggest that the capacity of Fru‐2,6‐P₂ to modulate partitioning of photoassimilate is an important determinant of growth and fitness in natural environments.     

Chloroplast lipid transfer processes in Chlamydomonas reinhardtii involving a TRIGALACTOSYLDIACYLGLYCEROL 2 (TGD2) orthologue

In plants, lipids of the photosynthetic membrane are synthesized by parallel pathways associated with the endoplasmic reticulum (ER) and the chloroplast envelope membranes. Lipids derived from the two pathways are distinguished by their acyl‐constituents. Following this plant paradigm, the prevalent acyl composition of chloroplast lipids suggests that Chlamydomonas reinhardtii (Chlamydomonas) does not use the ER pathway; however, the Chlamydomonas genome encodes presumed plant orthologues of a chloroplast lipid transporter consisting of TGD (TRIGALACTOSYLDIACYLGLYCEROL) proteins that are required for ER‐to‐chloroplast lipid trafficking in plants. To resolve this conundrum, we identified a mutant of Chlamydomonas deleted in the TGD2 gene and characterized the respective protein, CrTGD2. Notably, the viability of the mutant was reduced, showing the importance of CrTGD2. Galactoglycerolipid metabolism was altered in the tgd2 mutant with monogalactosyldiacylglycerol (MGDG) synthase activity being strongly stimulated. We hypothesize this to be a result of phosphatidic acid accumulation in the chloroplast outer envelope membrane, the location of MGDG synthase in Chlamydomonas. Concomitantly, increased conversion of MGDG into triacylglycerol (TAG) was observed. This TAG accumulated in lipid droplets in the tgd2 mutant under normal growth conditions. Labeling kinetics indicate that Chlamydomonas can import lipid precursors from the ER, a process that is impaired in the tgd2 mutant.