Circadian clock adjustment to plant iron status depends on chloroplast and phytochrome function

Plant chloroplasts are not only the main cellular location for storage of elemental iron (Fe), but also the main site for Fe, which is incorporated into chlorophyll, haem and the photosynthetic machinery. How plants measure internal Fe levels is unknown. We describe here a new Fe‐dependent response, a change in the period of the circadian clock. In Arabidopsis, the period lengthens when Fe becomes limiting, and gradually shortens as external Fe levels increase. Etiolated seedlings or light‐grown plants treated with plastid translation inhibitors do not respond to changes in Fe supply, pointing to developed chloroplasts as central hubs for circadian Fe sensing. Phytochrome‐deficient mutants maintain a short period even under Fe deficiency, stressing the role of early light signalling in coupling the clock to Fe responses. Further mutant and pharmacological analyses suggest that known players in plastid‐to‐nucleus signalling do not directly participate in Fe sensing. We propose that the sensor governing circadian Fe responses defines a new retrograde pathway that involves a plastid‐encoded protein that depends on phytochromes and the functional state of chloroplasts.     

X-ray and magnetic resonance stereotaxy for functional and nonfunctional neurosurgery

By means of new plastic stereotactic ring and head fixers, stereotactic procedures can be combined with MRI, with stereotactic coordinates obtained from the MRI images. The method was rechecked against CT stereotaxy and shows a good correspondence of the target coordinates. With MRI stereotaxy, structures near bony regions will be more accessible than with CT stereotaxy. Moreover, the MRI procedure seems to have advantages for functional therapy without the necessity of contrast ventriculography.     

Evidence for a ferredoxin-dependent choline monooxygenase from spinach chloroplast stroma

Chenopods synthesize betaine in the chloroplast via a two-step oxidation of choline: choline → betaine aldehyde → betaine. Our previous experiments with intact chloroplasts, and in vivo¹⁸O₂ labeling studies, led us to propose that the first step is mediated by a monooxygenase which uses photosynthetically generated reducing power (C Lerma, AD Hanson, D Rhodes [1988] Plant Physiol 88: 695-702). Here, we report the detection of such an activity in vitro. In the presence of O₂ and reduced ferredoxin, the stromal fraction from spinach (Spinacia oleracea) chloroplasts converted choline to betaine aldehyde at rates similar to those in intact chloroplasts (20-50 nanomoles per hour per milligram protein). Incorporation of ¹⁸O from ¹⁸O₂ by the in vitro reaction was demonstrated by fast atom bombardment mass spectrometry. Ferredoxin could be reduced either with thylakoids in the light, or with NADPH plus ferredoxin-NADP reductase in darkness; NADPH alone could not substitute for ferredoxin. No choline-oxidizing activity was detected in the stromal fraction of pea (Pisum sativum L.), a species that does not accumulate betaine. The spinach choline-oxidizing enzyme was stimulated by 10 millimolar Mg²⁺, had a pH optimum close to 8, and was insensitive to carbon monoxide. The specific activity was increased threefold in plants growing in 200 millimolar NaCl. Gel filtration experiments gave a molecular weight of 98 kilodaltons for the choline-oxidizing enzyme, and provided no evidence for other electron carriers which might mediate the reduction of the 98-kilodalton enzyme by ferredoxin.     

Mutant proteins of human interleukin 2. Renaturation yield, proliferative activity and receptor binding

Muteins, i.e. proteins altered by mutation of their genes, of interleukin 2 (Il2) were generated by oligonucleotide-directed mutagenesis in vitro. All acidic and basic residues conserved between man and mouse were exchanged as well as four lipophilic residues contained within four hydrophobic segments of the protein. The muteins were produced in Escherichia coli and submitted to a renaturation and purification protocol, before bioactivity and receptor binding of each of them was determined. All muteins besides two (K44/T125 and Q110/T125) could be renatured and purified. One mutein (K94/T125) exhibited a more than tenfold-improved renaturation yield. One amino exchange (Asp-20 to Asn) resulted in an about 20-fold reduction in proliferative activity and high-affinity receptor binding. The binding to the low-affinity Il2-binding protein (Tac antigen) was unimpaired. A second exchange (Arg-38 to Gln) had no effect on proliferative activity. The binding to both the high- and the low-affinity receptor, however, was reduced about 20-fold. Preliminary trials on the stability of these muteins by guanidinium hydrochloride denaturation studies detected no differences between wild-type interleukin 2 and muteins. It is suggested that Asp-20 forms part of the binding site for the large receptor subunit whereas Arg-38 is involved in the contact site to the small subunit.     

Characterization of cardiac innervation in the nudibranch,Archidoris montereyensis

Summary The heart of the nudibranch mollusc Archidoris montereyensis is regulated by a small number of powerful effector neurons located in the right pleural and visceral ganglia. Two identifiable neurons in the pleural ganglion, a heart excitor (pl_HE) and a heart inhibitor (Pl_HI), are especially important regulators of cardiac function in that low levels of spontaneous activity in either cell significantly alters the amplitude and rate of heart contractions. These neurons have extensive dendritic arbors within the right pleural ganglion and branching axonal processes within the visceral ganglion. The visceral ganglion also contains a heart excitor neuron (V_HE) and at least two heart inhibitor neurons (V_HI cells), but their influence on cardiac activity is weaker than that of the pleural ganglion cells. All of these heart effector cells appear to be motor neurons with axons that terminate predominately in the atrio-ventricular valve region of the heart via the pericardial nerve. The simplicity and strength of these neuronal connections to the heart of Archidoris make this a favorable preparation for studies of cardiac regulation.Abbreviations Pl _HE pleural ganglion heart excitor neuron - Pl _HI pleural heart inhibitor neuron - V _HE visceral ganglion heart excitor neuron - V _HI cells, visceral heart inhibitor neurons - V _K visceral kidney excitor neuron - V _G visceral gill excitor neuron