Prescribed burning is a major control over element cycles in Tallgrass prairie (Eastern Kansas, USA). In this paper we report potential effects of fire on nonsymbiotic nitrogen fixation. Fire resulted in additions of available P in ash, which may stimulate nitrogen fixation by terrestrial cyanobacteria. Cyanobacterial nitrogenase activity and biomass responded positively to additions of ash or P in laboratory assays using soil. Further assays in soil showed that cyanobacteria responded to changes in available N:available P ratio (aN:P) across a range of concentrations. Nitrogen fixation rate could be related empirically to aN:P via a log-linear relationship. Extrapolation of laboratory results to the field yielded a maximal estimate of 21 kg N ha y. Results support arguments from the marine and terrestrial literature that P availability is central to regulation of ecosystem N budgets.

Five tripeptides with the sequence L-Arg-D-X-L-Phe showed antibiotic activity on fungi and on some pathogenic moulds. The action of the peptides could be neutralized in the cross-strip test by the L-amino acid corresponding to the D-amino acid in the middle position.

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The investigation of the sequences Arg--Gly-Phe and Arg-Gly-Phe-Phe (B 22-25), which possess an insulin-like effect on their ownt 2 !, and of similar B-chain sequences was continued. Using conventional methods and Merrifield's solid phase method, peptides containing all or part of the sequence Gly-Glu-Arg-Gly-Phe-Phe-Tyr-Ala were synthesized. The C-terminal ends of the peptides were converted to amides or methyl esters or left as free car boxy 1 groups. The peptides were administered by intraperitoneal injection to rats together with [U-14 C]gIucose. The incorporation of radioactivity into diaphragm and other tissues was measured. The sequences Arg-Gly-Phe-and Arg-Gly-Phe--Phe stimulate 14 C incorporation in diaphragm and in epididymal fat tissue. The activity decreases in the order amides > methyl esters > free carboxyl compounds. The peptides with two phenylalanine residues are much more effective than the peptides possessing only one phenylalanine residue. The peptide Arg-Gly-Phe-Phe-NH2 for example, is able to increase the 14 C incorporation in diaphragm to a level comparable with maximal stimulation by insulin.-The mono-7V a -acetylation of Arg-Gly--Phe-Phe-NH2 does not decrease its insulin-like activity. The glycyl residue B 23 is necessary for biological activity. Unlike Arg-Gly-Phe-NH2, the peptide Arg-Phe-NH2 has no effect. The peptide Arg-Gly-Phe-Phe-Tyr-Ala-OH and its C-terminal amide, which contain tyrosine (B 26), had approximately the same effects as Arg-Gly--Phe-Phe-NH2 on the diaphragm. In epididymal fat tissue, the tyrosine-containing peptides were approximately twice as active as Arg-Gly-Phe-Phe-NH 2 . Addition of glutamic acid (B 21) and glycyl-glutamic acid (B20 -21) to the B 26 tyrosine-containing sequence produced synthetic peptides nearly insoluble in water. Their activity could not be measured in our test systems. When the diaphragm was preincubated with trypsin in vitro, the action of insulin was abolished for 20-30 min. During this time the effects of Arg-Gly-Phe-NH 2 and Arg-Gly-Phe-Phe-NH2 on glucose uptake and glycogen content were unchanged. From this it is concluded that the activity of the peptides is independent of the insulin receptor. Struktur und Wirkung von Insulin, XII: Weitere Untersuchungen an biologisch aktiven synthetischen Fragmenten der B-KetteZusammenfassung: Die Untersuchung der Sequen-welche schon für sich allein Insulinaktivität bezen Arg-Gly-Phe und Arg-Gly-Phe-Phe (B 22-25), sitzen&\ und ähnlicher B-Kettensequenzen wurde Abbreviations: Boc=tert. butyloxycarbonyl; Z = benzyloxycarbonyl.

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