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Legumes play a crucial role in nitrogen supply to grass-legume mixtures for ruminant fodder. To quantify N transfer from legumes to neighbouring plants in multi-species grasslands we established a grass-legume-herb mixture on a loamy-sandy site in Denmark. White clover (Trifolium repens L.), red clover (Trifolium pratense L.) and lucerne (Medicago sativa L.) were leaf-labelled with 15 N enriched urea during one growing season. N transfer to grasses (Lolium perenne L. and xfestulolium), white clover, red clover, lucerne, birdsfoot trefoil (Lotus corniculatus L.), chicory (Cichorium intybus L.), plantain (Plantago lanceolata L.), salad burnet (Sanguisorba minor L.) and caraway (Carum carvi L.) was assessed. Neighbouring plants contained greater amounts of N derived from white clover (4.8 gm ). Grasses having fibrous roots received greater amounts of N from legumes than dicotyledonous plants which generally have taproots. Slurry application mainly increased N transfer from legumes to grasses. During the growing season the three legumes transferred approximately 40 kg N ha -1 to neighbouring plants. Below-ground N transfer from legumes to neighbouring plants differed among nitrogen donors and nitrogen receivers and may depend on root characteristics and regrowth strategies of plant species in the multi-species grassland.

To identify the region(s) of the insulin receptor and the insulin-like growth factor I (IGF-I) receptor responsible for ligand specificity (high-affinity binding), expression vectors encoding soluble chimeric insulin/IGF-I receptors were prepared. The chimeric receptors were expressed in mammalian cells and partially purified. Binding studies revealed that a construct comprising an IGF-I receptor in which the 68 N-terminal amino acids of the insulin receptor a-subunit had replaced the equivalent IGF-I receptor segment displayed a markedly increased affinity for insulin. In contrast, the corresponding IGF-I receptor sequence is not critical for high-affinity IGF-I binding. It is shown that part of the cysteine-rich domain determines IGF-I specificity. We have previously shown that exchanging exons 1, 2, and 3 of the insulin receptor with the corresponding IGF-I receptor sequence results in loss of high affinity for insulin and gain ofhigh affinity for IGF-I. Consequently, it is suggested that the ligand specificities of the two receptors (i.e., the sequences that discriminate between insulin and IGF-I) reside in different regions of a binding site with common features present in both receptors.The polypeptide hormone insulin plays an essential role in metabolic regulation (1-3). It exerts its physiological effects through its interaction with a specific high-affinity receptor, which is an integral cell surface membrane glycoprotein with a relative molecular mass of about 400 kDa (4-6). The receptor is a disulfide-linked heterotetramer, made up of two a-and two B-subunits (7-12). Its ligand binding domains are formed by the extracellular a-subunits, whereas the /3-subunits consist of a short extracellular domain, a single transmembrane segment, and an intrinsic intracellular tyrosine protein kinase domain involved in signal transduction (13-15).The insulin-like growth factor I (IGF-I) receptor shows extensive similarity to the insulin receptor in amino acid sequence, domain structure, and signaling mechanism (11,16). Although insulin and IGF-I are very similar in amino acid sequence, they bind only weakly to the receptor for the other hormone (17,18).Ligand binding affinity and specificity are central to receptor activation, regulation, and function. Chimeric molecules comprising parts of related receptors have been useful for elucidating relationships between structure and specificity in several systems (e.g., refs. 19-21). In a previous study we showed that it is possible to convert the insulin receptor into a receptor with high affinity for IGF-I and low affinity for insulin by substituting an insulin receptor cDNA fragment (exons 1, 2, and 3) with that for the corresponding . This suggested to us that the ligand specificity of both receptors resides in the same region. Here, we extend this approach to a more detailed analysis of the ligand specificity of this receptor family. The first 68 N-terminal amino acids of the insulin receptor are found to determine insulin specificity. The corresponding 62 N-t...

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