Itt

642 effect appears to be achieved, as for other peptide hormones, through a second messenger system, namely, cyclic AMP and calcium ions.

By contrast, 20-HE and JH are lipophilic and thus able to move through the cell membrane to the nuclear membrane where they bind to specific receptors. In Drosophila and Manduca the receptor for 20-HE has been identified and characterized as a protein heterodimer (Hall, 1999; Riddiford et al., 1999). Its two components are EcR (ecdysone receptor) and USP (ultraspiracle*), both of which may exist in slightly different forms and concentrations. The 20-HE binds to the EcR part of the dimer, and while the function of USP remains unclear, both components of the dimer are necessary to transport the hormone to the chromosomes for gene activation (Lezzi et al., 1999). Precisely how 20-HE works is not known; the most widely accepted view is that the hormone, like the steroid hormones of vertebrates, induces a gene-activation cascade. The 20-HE activates so-called "early" genes that encode regulatory proteins. The latter, in turn, modulate the activity of "middle," and eventually "late," genes whose transcriptional products carry out the appropriate tissue-specific process (Riddiford, 1985; Doctor and Fristrom, 1985). The pathways of gene activation that are followed depend on the 20-HE concentration and receptor isoform; as well, they will be species- and tissue-specific, resulting in the numerous effects induced by this hormone.

The site and mode of action of JH are less clear, a statement that is perhaps not surprising given its multifunctional role in insects. However, with respect to postembryonic development where it qualitatively modifies the effects of 20-HE, JH is generally assumed to act at the same level; that is, it influences gene activation and messenger RNA synthesis. Elucidation of its mode of action has been hampered by the inability to characterize its receptors. The proposal of Jones and Sharp (1997) that USP is a receptor for JH in Drosophila is intriguing but requires extension to other insects. One suggestion for the manner in which JH works is that it may alter the conformation of the chromatin so that 20-HE can activate only larva-specific genes.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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