Synthesis and inhibition of macronuclear DNA in regenerating Blepharisma

Abstract

AbstractThe macronuclear cycle in regenerating posterior fragments of albino Blepharisma intermedium consists of four sequential stages which result in the reversible condensation of the macronucleus, and which are similar to those in the pigmented strain. Tritiated deoxyuridine incorporation occurs predominantly during the first three hours of regeneration and then declines. Of the early hours, hour 2 exhibits a significant increase in the proportion of heavily‐labeled cells. A statistical comparison of whole organisms labeled through exposure to radioactive precursor for a one‐hour period with fragments labeled at each hour of regeneration reveals a significantly greater number of labeled fragments for each of hours 1 through 4. Asynchronous patterns of DNA replication are evident in the macronucleus of both regenerating and whole cells. Oval‐shaped pellicular rings, consisting of dense accumulations of silver halide grains are evident during hours 5 and 6 of regeneration.Continuous inhibition of DNA synthesis during regeneration produces effects which depend upon the specific inhibitor employed. 5‐Aminouracil (AU) effects an appreciable delay of regeneration, without lethal or abnormal consequences. AU, hydroxyurea (HU), 5 bromodcoxyuridine (BUdR) and 5‐fluorodeoxyuridine (FUdR) exhibit a marked inhibitory effect upon division when they are administered to two‐hour fragments. The first three hours of regeneration is the period of greatest susceptibility to inhibitor action with respect to post‐regeneration division. AU, BUdR and FUdR interfere with the sequence of macronuclear events when these inhibitors are administered during hours 2–4 of regeneration. Contracted macronuclei are present at 24 hours and are related to division delay. HU, FUdR and BUdR treatment, during the later hours of regeneration, results in macronuclear fragmentation, branching and abnormal massing, respectively. Cytoplasmic abnormalities result from fragment exposure to the inhibitors during hours 4–6, with BUdR producing the most pronounced effects.