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Bifunctional Cross-linking Reagents and the Fine Structure of Protein in Ehrlich Ascites Tumor Cell Nuclei¹

( Institute of Cellular Biology, University of Connecticut, Storrs, Connecticut)

Two bifunctional reagents, p,p'-difluoro-m,m'-dinitrodiphenyl sulfone and 1,5-difluoro-2,4-dinitrobenzene, have been utilized for the selective preservation of protein in frozen-dried cells and, despite their low density, for the production of useful image contrast in the electron microscope. Interphase nucleoli consist of an anastomotic framework of dense, protein-rich fibrils within whose interstices occurs a profusion of discrete particles measuring about 40 Å together with less numerous, larger particles corresponding to ribosomes. Discrete 40-Å particles also occur intermittently in catenulate series continuous with the fibrils. Many interphase nucleoli are characterized by prominent vacuole-like spaces which abound in 40-Å particles. The mitotic chromosome consists of a fibrillar protein network, enclosing 40-Å particles much as in the nucleolus; the density of this network is too high to permit a study of its morphologic relationship with fine particles. Directness of localization of protein structure, without prior enzymatic removal of the basophilia normally obscuring it, is an attribute of the bifunctional reagents which usefully complements their property of cross-linking side chains of the aminoacyl residues of peptides.

¹ This work was supported by a research grant (GM-08326) and a Research Career Development Award from the National Institutes of Health, USPHS. Contribution No. 117 from the Institute of Cellular Biology.

Received 9/ 7/64. Revised 3/10/65.