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Development and Growth Regulation

Binding of Spermidine to a Unique Protein in Thin-Layer Tobacco Tissue Culture ¹

Department of Biology, Yale University, New Haven, Connecticut 06511, West Haven Veterans Administration Medical Center, West Haven, Connecticut 06516, Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510

The mechanism by which spermidine induces the appearance of floral buds in thin-layer tobacco (Nicotiana tabacum) tissue culture was studied by following the fate of the radioactive compound. [³H]Spermidine was taken up rapidly by the tissue, and after a brief lag, a portion was bound to trichloroacetic acid precipitable macromolecules. Such binding increased to a maximum on day 4 of culture, coinciding with the onset of bud differentiation, and declined thereafter until shortly before flowering. About 82% of the label in the trichloroacetic acid precipitate remained as spermidine, 14% was metabolized to putrescine, 3% to spermine, and 1% to -aminobutyric acid. Spermidine was covalently bound to a protein with a molecular size of about 18 kilodaltons. Hydrolysis of this protein and analysis of the labeled entities revealed 81% spermidine, 16% putrescine, and 3% spermine. This post-translational modification of a unique protein by attachment of spermidine may be causally connected to the appearance of flower buds in thin-layer tobacco cultures.

¹ Supported by National Science Foundation grant DMB-8504248 to A. W. G.

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