Fluorescence induction in the phycobilisome-containing cyanobacterium Synechococcus sp PCC 7942: Analysis of the slow fluorescence transient

At room temperature, the chlorophyll (Chl) a fluorescence induction (FI) kinetics of plants, algae and cyanobacteria go through two maxima, P at ∼ 0.2-1 and M at ∼ 100-500 s, with a minimum S at ∼ 2-10 s in between. Thus, the whole FI kinetic pattern comprises a fast OPS transient (with O denoting origin) and a slower SMT transient (with T denoting terminal state). Here, we examined the phenomenology and the etiology of the SMT transient of the phycobilisome (PBS)-containing cyanobacterium Synechococcus sp PCC 7942 by modifying PBS → Photosystem (PS) II excitation transfer indirectly, either by blocking or by maximizing the PBS → PS I excitation transfer. Blocking the PBS → PS I excitation transfer route with N-ethyl-maleimide [NEM; A. N. Glazer, Y. Gindt, C. F. Chan, and K.Sauer, Photosynth. Research 40 (1994) 167-173] increases both the PBS excitation share of PS II and Chl a fluorescence. Maximizing it, on the other hand, by suspending cyanobactrial cells in hyper-osmotic media [G. C. Papageorgiou, A. Alygizaki-Zorba, Biochim. Biophys. Acta 1335 (1997) 1-4] diminishes both the PBS excitation share of PS II and Chl a fluorescence. Here, we show for the first time that, in either case, the slow SMT transient of FI disappears and is replaced by continuous P → T fluorescence decay, reminiscent of the typical P → T fluorescence decay of higher plants and algae. A similar P → T decay was also displayed by DCMU-treated Synechococcus cells at 2 °C. To interpret this phenomenology, we assume that after dark adaptation cyanobacteria exist in a low fluorescence state (state 2) and transit to a high fluorescence state (state 1) when, upon light acclimation, PS I is forced to run faster than PS II. In these organisms, a state 2 → 1 fluorescence increase plus electron transport-dependent dequenching processes dominate the SM rise and maximal fluorescence output is at M which lies above the P maximum of the fast FI transient. In contrast, dark-adapted plants and algae exist in state 1 and upon illumination they display an extended P → T decay that sometimes is interrupted by a shallow SMT transient, with M below P. This decay is dominated by a state 1 → 2 fluorescence lowering, as well as by electron transport-dependent quenching processes. When the regulation of the PBS → PS I electronic excitation transfer is eliminated (as for example in hyper-osmotic suspensions, after NEM treatment and at low temperature), the FI pattern of Synechococcus becomes plant-like.     

Genetic control of DNA initiation in Escherichia coli

We describe the isolation, and properties of a mutant (CT28) of Escherichia coli with a temperature-sensitive defect in DNA initiation that is reversible. The mutation (dna-28) responsible for this defect is shown to be located in the same region of the map as the dnaC group of DNA initiation mutants.A terminalized culture of CT28 initiates DNA synthesis synchronously immediately upon lowering the temperature, and will do so in the presence of chloram-phenicol.During prolonged incubation at the non-permissive temperature, the cells accumulate a capacity to initiate multiple rounds of replication per chromosome.The variation in the susceptibility of the argH^? and thyA^? alleles to reversion by pulse mutagenesis with nitrosoguanidine during a synchronous round of DNA replication, suggests that this round of replication is bidirectional and commences from an origin in the vicinity of 60 to 65 minutes.CT28 contains two temperature-sensitive mutations. These have been mapped and separated into two derivative strains. One of these, CT28-3b, carries the dna-28 mutation of the C locus, and like the parental double mutant is reversibly temperature-sensitive for an initiation function; but it is more temperature-sensitive than either the double mutant or the other single mutant derivative, CT28-1. The other, CT28-1, is not defective in DNA replication or initiation of replication at the non-permissive temperature.     

Evidence for a secretion of thyroxine by isolated hog thyroid cells

Isolated thyroid cells prepared from hog thyroid glands by tryptic dispersion were incubated with ¹³¹I⁻ for 1–6 h. Free [¹³¹I]thyroxine was identified in the incubation medium by three chromatographic methods. Neither [¹³¹I]iodotyurosines nor [¹³¹I]triiodothyronine were detected. The [¹³¹I]thyroxine released in the medium by 100 μl of cells (packed cell volume) after a 6-h incubation period amounted to 1.16% (S.E. = ± 0.39) of the total radioactivity. The medium [¹³¹I]thyroxine represented 15–25% of the total [¹³¹I]thyroxine synthesized during the 6 h of incubation. Thyrotropin, 1–60 munits/ml, increased the medium [¹³¹I]thyroxine content 2–4 fold. Dibutyryl cyclic AMP mimicked the effect of thyrotropin. The amount of medium [¹³¹I]thyroxine was strictly related to the amount of incubated cells but was independent of the volume of the incubation medium. When prelabeled cells were incubated in the presence of methimazole the increase in medium [¹³¹I]thyroxine was quantitatively related to a decrease in the intracellular [¹³¹I]thyroxine. Addition of dinitrotyrosine, an inhibitor of the deiodinase activity, induced the release of iodotyrosines in the incubation medium. That the incubation supernatant of isolated thyroid cells did contain free thyroxine but no iodotyrosines suggests that the normal mechanisms of proteolysis of thyroglobulin and deiodination of iodotyrosines inside the cells are preserved. From these data, it was concluded that the thyroxine release by isolated cells represents a real secretion.     

An Unexpected Major Groove Binding of Netropsin and Distamycin A to tRNAphe

Abstract

Crystalline complexes of yeast tRNA^phe and the oligopeptide antibiotics netropsin and distamycin A were prepared by diffusing drugs into crystals of tRNA. X-ray structure analyses of these complexes reveal a single common binding site for both drugs which is located in the major or deep groove of the tRNA T-stem. The netropsin-tRNA complex is stabilized by specific hydrogen bonds between the amide groups of the drug and the tRNA bases G51 0(6), U52 0(4) and G53 N(7) on one strand, and is further stabilized by electrostatic interactions between the positively charges guanidino side chain of the drug and the tRNA phosphate P53 on the same strand and the positively charged amidino propyl side chain and the phosphates P61, P62 and P63 on the opposite strand of the double helix. These results are in contrast to the implicated minor groove binding of these drugs to non-guanine sequences in DNA. The binding to the GUG sequence in tRNA implies that major groove binding to certain DNA sequences is possible.     

Interferon (IFN)-like antiviral effect is induced by unspecific cross-linking of cell surface receptors

Photosystem I preparations were irradiated with UV to destroy vitamin K₁ in situ. The depletion of vitamin K₁ resulted in inactivation of NADP⁺ photoreduction and introduction of a 220 ms component in the flash generated P700⁺ re-reduction at room temperature. The photoreduction of the terminal FeS centers FA and FB in control and vitamin K₁-depleted preparations at 7 K were comparable. The data confirm that vitamin K₁ is functionally implicated in primary electron transfer reactions in PS I at physiological temperature, and that the anomalous results at cryogenic temperature may be explicable in terms of a by-pass of the vitamin K₁ acceptor site or heterogeneity introduced into the photosystem by quinone removal.     

Cell division and DNA topisomerase I activity in root meristems of pea seedlings during water stress

ABSTRACT

Low water potential, generated by PEG addition to the liquid medium of hydroponically grown pea seedlings, induces a fall in moisture content in the roots, followed by the arrest of elongation. This water stress reduces the mitotic index of root meristems during the treatment and induces the appearance of a peak of mitosis at 12 hours from the beginning of recovery. This peak suggests that during water stress the cell cycle is blocked in G2 or late S phase. In a first attempt to understand the biochemical events leading to cell cycle arrest, we tested the in vitro activity of DNA topoisomerase I extracted from stressed or control root meristems. The activity of this enzyme in extracts from stressed seedlings was lower than in controls, whereas it was higher in extracts from seedlings which had recovered from water stress for a few hours. The highest specific activity was observed with seedlings at 24 hours from the start of recovery. The fact that during stress treatments and recovery there was no variation in the synthesis of a 45 kDa protein, indicated as DNA topoisomerase I, suggested that the activity of this enzyme could be posttranslationally regulated. The hypothesis that variations in the concentration of unknown endogenous regulators of the activity of this enzyme may take place during water loss or uptake in the cytosol of meristematic cells is discussed.