Differences in protein content of sister nuclei: evidence from binucleate and mononucleate cells

DNA and protein contents of pairs of sister nuclei were determined using a combined Feulgen-dinitrofluorobenzene technique. Sister nuclei were studied in binucleate cells, induced by treatment with 0.1% caffeine, and in sister mononucleate cells of untreated roots. Excised pea roots, grown in culture, were treated with 5-aminouracil to induce mitotic synchrony and with caffeine at the time of peak mitotic index, to provide the maximum number of binucleate cells. The induced binucleate cells form a marked population which was followed through a cell cycle; sister nuclei showed a correlation of volume and protein content, r = 0.79. Protein contents of sister nuclei were rarely identical and at 1 + 2 and 1 + 6 h the difference in protein contents of sister nuclei was significant (p = 0.05). Mean nuclear protein content decreased from 1 + 2 to 1 + 6 h; then, as nuclei entered S phase, their protein content increased. From 1 + 2 to 1 + 14 h the increase in protein content, in absolute amount, was identical in both sister nuclei. This suggests that there was a biphasic pattern of protein uptake; it is differential, in sister nuclei, in the first part of G1 but is identical throughout the rest of interphase. Analysis of sister nuclei from sister mononucleate cells showed a similar pattern of change; this is further evidence, from untreated cells, of a biophasic pattern of protein uptake. Caffeine-treated nuclei had lower protein contents than untreated nuclei, yet they completed a cell cycle and entered mitosis; this suggests that nonessential proteins were no longer present. It is proposed that mitosis is asymmetrical for molecules that regulate rates of macromolecular synthesis, cell growth, and progress through a cell cycle and that once the initial asymmetry has been established, it is maintained throughout interphase, even in binucleate cells in which the two nuclei share a common cytoplasm.