Hopanes are abundant in ancient sedimentary rocks at discrete intervals in Earth history, yet interpreting their significance in the geologic record is complicated by our incomplete knowledge of what their progenitors, hopanoids, do in modern cells. To date, few studies have addressed the breadth of diversity of physiological functions of these lipids and whether those functions are conserved across the hopanoid-producing bacterial phyla. Here, we generated mutants in the filamentous cyanobacterium, Nostoc punctiforme, that are unable to make all hopanoids (shc) or 2-methylhopanoids (hpnP). While the absence of hopanoids impedes growth of vegetative cells at high temperature, the shc mutant grows faster at low temperature. This finding is consistent with hopanoids acting as membrane rigidifiers, a function shared by other hopanoid-producing phyla. Apart from impacting fitness under temperature stress, hopanoids are dispensable for vegetative cells under other stress conditions. However, hopanoids are required for stress tolerance in akinetes, a resting survival cell type. While 2-methylated hopanoids do not appear to contribute to any stress phenotype, total hopanoids and to a lesser extent 2-methylhopanoids were found to promote the formation of cyanophycin granules in akinetes. Finally, although hopanoids support symbiotic interactions between Alphaproteobacteria and plants, they do not appear to facilitate symbiosis between N. punctiforme and the hornwort Anthoceros punctatus. Collectively, these findings support interpreting hopanes as general environmental stress biomarkers. If hopanoid-mediated enhancement of nitrogen-rich storage products turns out to be a conserved phenomenon in other organisms, a better understanding of this relationship may help us parse the enrichment of 2-methylhopanes in the rock record during episodes of disrupted nutrient cycling.
Cyanobacterial lipid droplets (LDs) are packed with hydrophobic energy-dense compounds and have great potential for biotechnological expression and the compartmentalization of high value compounds. Nostoc punctiforme normally accumulates LDs containing neutral lipids, and small amounts of heptadecane, during the stationary phase of growth. In this study, we further enhanced heptadecane production in N. punctiforme by introducing extrachromosomal copies of aar/adc genes, and report the discovery of a putative novel lipase encoded by Npun_F5141, which further enhanced alkane production. Extra copies of all three genes in high light conditions resulted in a 16-fold higher accumulation of heptadecane compared to the wild type strain in the exponential phase. LD accumulation during exponential phase also increased massively to accommodate the heptadecane production. A large number of small, less fluorescent LDs were observed at the cell periphery in exponential growth phase, whereas fewer number of highly fluorescent, much larger LDs were localized towards the center of the cell in the stationary phase. These advances demonstrate that cyanobacterial LDs are an ideal model platform to make industrially relevant compounds, such as alkanes, during exponential growth, and provide insight into LD formation in cyanobacteria.
Fibrin cross-linkage results when a plasma zymogen, Factor XIII, is activated by thrombin and calcium to form an enzyme, transglutaminase, which makes a-a and y-y cross-links in fibrin. During the past decade attempts have been made by several investigators to study the structure of Factor XIII, the inactive precursor of the plasma transglutaminase. Although the nature of the reaction catalyzed by this enzyme and its presence in human plasma and platelets have been established without much disagreement, the molecular properties of this protein have never been agreed upon by independent investigators. Consequently, there is ample discrepancy in the literature regarding the molecular weight and the subunit structure of Factor XIII.In 1961, Lorand, utilizing a partially purified plasma Factor XI11 estimated a molecular weight of 130,000 and showed by means of the ultracentrifuge that Factor XI11 aggregates in the absence of SH compounds.7 In the same year, Loewy and coworkers independently used a DEAE-cellulose purification method and found the Factor XI11 protein to have a molecular weight of 340,000 with three subunits of 110,000 each, which Loewy felt were inactive.6 In 1964, Lewis found the protein to have a molecular weight of 200,000.5 In 1970, Keisselbach and coworkers utilized a modification of Loewy's procedure and found the molecular weight to be 330,000.
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