Cytochemistry of macronuclear reorganization

Abstract

The reorganization phenomenon in the macronuclear was related to the cell life cycle of Euplotes. At generation times of 20 hr the G1 period occupied 5 hr, the S period 12.5 hr and the G2 period 2.5 hr. Signs of cytoplasmic division were present during the last third of the G2 period. The S phase consisted of two phases. During the first 80 per cent of the S phase (S1 phase) DNA synthesis was slow. When only 20 per cent of the S phase remained the bands had travelled approximately 45 per cent of the length of the macronucleus. During the last 20 per cent of the S phase (S2 phase) the rate of DNA synthesis was approximately 4 times as rapid as during the first part. As generation time increased, the length of the S phase also increased. In slow-growing organisms some macronuclei exhibited reorganization bands but no incorporation of 3H-thymidine whereas the majority of reorganization bands did incorporate this isotope. Macronuclei from Euplotes were found by cytochemical techniques to undergo quantitative changes in their macromolecular composition during the reorganization phenomenon. The reorganization phenomenon consisted not only of a duplication of the DNA component as previously described by Gall but also of a sudden increase in macronuclear protein. Microinterferometry indicated that in macronuclei isolated from rapidly multiplying organisms a doubling of almost all nuclear macromolecules took place in the reorganization band. The dry mass changes observed could be due only to a small extent to a doubling in DNA content since DNA made up less than 10 per cent of the dry mass material. Autoradiography with 3H-lysine and 3H-arginine showed that zones distal to the reorganization bands became strongly labelled but that protein labelling seemed to occur independently of reorganization throughout the duplicated and unduplicated segments of the macronucleus. The cytochemical characteristics of the forward and rear zones of the reorganization bands are described.