Lateral Transport of Ions into the Xylem of Corn Roots: II. Evaluation of a Stelar Pump

When an excised corn (Zea mays) root pretreated with chloride was exposed for 10 minutes to pulse labeling with ³⁰Cl and then transferred to unlabeled chloride, the activity in the xylem exudate reached a maximum about 4 minutes after pulse labeling was discontinued and then declined sharply. The rate at which labeled chloride was transported across the root into the xylem and basipetally therein was on the order of 75 to 250 centimeters per hour. Consequently, symplasmic movement of chloride in corn roots is fast and may not be rate-limiting in transfer from the root surface to the xylem. Experiments on pulse labeling with ²²Na gave similar results. A large fraction of the absorbed ²²Na was not translocated into the exudate but was tightly sequestered in a cell compartment, probably the vacuole.     

Two categories of c/c ratios for higher plants

¹³C/¹²C ratios have been determined for plant tissue from 104 species representing 60 families. Higher plants fall into two categories, those with low δ_PDBI¹³C values (—24 to —34‰) and those with high δ ¹³C values (—6 to —19‰). Algae have δ ¹³C values of —12 to —23‰. Photosynthetic fractionation leading to such values is discussed.     

Investigation of huge negative circular dichroism spectra of some nucleoproteins

Under certain conditions of preparation, DNA, whether free or complexed with polylysine or histone KAP (I, fl), produce huge negative circular dichroism (CD) spectra with maxima at about 270nm. In order to investigate the cause of these spectra, reconstituted polylysine-DNA complex was used as a model system. It was found that the CD change of DNA in the complex is not a linear function of the fraction of base pairs bound. Such a CD spectrum is not changed despite dilution up to 128 folds for as long as 12 hours. Difference CD spectra taken between free DNA and any of the complexes are qualitatively the same, and are similar to those of free DNA and nucleohistone KAP (Fasman et al., Biochemistry 9, 2814-2822, 1970), free DNA and direct mixed polylysine-DNA complexes, or free DNA in high salt (Chang et al., Biochemistry12, 3028-3032, 1973). The suggestion is made that this CD spectrum might be caused by specific conformational changes in DNA, perhaps belonging to the family of B to C transitions followed by a further structural distortion of DNA due to aggregation of the nucleoprotein molecules.     

Copper levels in cervical mucus during normal menstrual cycles

To establish the pattern of copper levels in cervical mucous, individual samples were taken from 10 normal women (aged 23-46), 2-3 times per week during the cycle. Analysis using a modification of the Stoner and Dasler wet ash microtechnique showed the lowest amounts of copper per 100 mg of dried ash (mean-3.2 mcg) in the ovulatory phase. The highest amount of copper was found in the secretory phase (mean 19.4 mcg). The copper level seemed to vary directly with the level of endogenous progesterone and inversely with the level of endogenous estrogen. No significant correlation between age and amount of copper secreted could be established because of the small size of the study.     

Remnants of trees from archaeological finds in Bohemia,I

The present paper gives an account of the archaeobotanical results of charcoal investigations from the lower valley of the river Oh?e (Bohemia). According to the analysis the primal vegetation may be the allianceCarpinion betuli.     

Cation Transport in Escherichia coli : VII. Potassium requirement for phosphate uptake

When Escherichia coli K-12 is grown in media containing limiting amounts of K, growth continues normally until all the extracellular K has been consumed. Thereafter the rates of growth, glucose consumption, and oxygen consumption decrease progressively, and the cell contents of K and P fall. These changes, referred to as K limitation, are all reversed by the addition of K. By specifically altering the ionic composition of the cells it was shown that these metabolic disturbances are not due to changes in the cell content of K or Na, but are directly related to the absence of K from the extracellular medium. The cell pool of inorganic P and the uptake of PO₄ from the medium are low in K-limited cells and are immediately stimulated by the addition of K, suggesting that the primary effect of K limitation is to inhibit PO₄ uptake. All the metabolic effects of K limitation can be attributed to inhibition of PO₄ uptake. The requirement of extracellular K for PO₄ uptake may be due to a coupling between the uptake of K and PO₄.     

A method for estimating the number of invariant amino acid coding positions in a gene using cytochrome c as a model case

This paper shows, within the limitations of the assumption stated below, that approximately 27–29 of the unmutated codons which determine the amino acids of cytochrome c are invariant because of biological requirements. A mutation is defined here as the change of a single base in the sequence of a trinucleotide codon, which change alters the amino acid coded for. Codons, if any, in which mutations would be vigorously selected against are termed invariant codons. We assume that, subject to one adjustment, those mutations in the cytochrome c gene which survived in the descent of today's species are randomly distributed among the variable codons. The one adjustment arises from the possibility that a very few codon positions may exhibit frequencies of mutation sufficiently great to justify the exclusion of these codons from the overall distribution on the grounds that the frequency of mutation occurring in these few positions is clearly inconsistent with the assumption of randomness. There are 5 out of the total 110 codons in the cytochrome c structural gene which have clearly sustained an abnormally large number of mutations.This project received support from grants to W.M.F. from the National Institutes of Health (NB-04565) and the National Science Foundation (GB-4017).     

Sodium absorption by barley roots: role of the dual mechanisms of alkali cation transport

Radioactively labeled Na(+) absorbed by barley roots was sequestered in an intracellular compartment or compartments ("inner" spaces) in which it was only very slowly exchangeable with exogenous Na(+). Absorption of this fraction proceeded at a constant rate for at least 1 hour.When the rate of Na(+) absorption was examined over the range of concentrations, 0.005 to 50 mm, the isotherm depicting the relation showed dual kinetics as follows. Over the range, 0.005 to 0.2 mm, a single Michaelis-Menten term describes the relation between the concentration of Na(+) and the rate of its absorption. The mechanism of Na(+) absorption operating over this range of concentrations, mechanism 1 of alkali cation transport, is severely inhibited in the presence of Ca(2+) and virtually rendered inoperative for Na(+) transport by the combined presence of Ca(2+) and K(+). The mechanism is equally effective in Na(+) transport whether Cl(-) or F(-) is the anion, but is somewhat inhibited when the anion is SO(4) (2-).Over the high range of concentrations, 0.5 to 50 mm Na(+), a second, low-affinity mechanism of Na(+) absorption comes into play. In the presence of Ca(2+) and K(+), this mechanism 2 is the only one to transport Na(+) effectively, since Na(+) absorption via mechanism 1 is virtually abolished under these conditions.Anaerobic conditions, low temperature, and the uncoupler, 2,4-dinitrophenol, inhibit Na(+) absorption both at low and high Na(+) concentrations.