The syntheses of deoxyribonucleic acid and histone in the onion root meristem

Is human histone gene expression autogenously regulated?

It has been well documented that core and H1 histone mRNAs accumulate in a manner which closely parallels the initiation of DNA synthesis and histone protein synthesis, suggesting that the onset of histone gene expression early during S phase is at least in part transcriptionally mediated. In fact, it appears that throughout S phase the synthesis of histone proteins is modulated by the availability of histone mRNAs. On the other hand, the stability of histone mRNAs and the destabilization of histone mRNAs when DNA replication is completed or inhibited are highly selective, tightly coupled and largely post-transcriptionally controlled. We present a model to account for histone mRNA turnover whereby the natural or inhibitor-induced termination of DNA replication results in an immediate loss of high affinity binding sites for newly synthesized histone proteins which in turn brings about a transient accumulation of unbound histones. These unbound histones could modify the histone translation complex, via interactions with polysomal histone mRNAs, in such a manner as to render histone mRNAs accessible to cellular ribonucleases. This type of mechanism would be operative solely at the post-transcriptional level and would be compatible with the rapid, RNA synthesis-independent destabilization of histone mRNAs which occurs following inhibition of DNA replication, as well as with the requirement for protein synthesis for histone mRNA destabilization to be initiated.

In the higher plant Pisum sativum maturation of nascent DNA is blocked by cycloheximide, but only after 4-8 replicons are joined

Velocity sedimentation in alkaline sucrose gradients, single cell autoradiography and cytophotometry were used to determine if protein synthesis is required for the maturation of nascent replicons to chromosomal-sized molecules in cultured pea-root cells. The results obtained showed that cycloheximide at 5 and 10 microgram/ml, added either before or during labeling with tritiated thymidine, blocked maturation of nascent DNA at an intermediate size of 72-140 X 10(6) daltons single-stranded DNA. To reach this size, nascent replicons - which are 18 X 10(6) daltons single-stranded DNA each - were replicated and groups of 4-8 replicons were joined even though protein synthesis was reduced to 15% of the control. Further maturation of the nascent molecules to chromosomal size, however, was prevented and this resulted in the accumulation of nascent molecules in the 72-140 X 10(6) daltons range. The experiments also showed that the joining of nascent replicons is not an absolute function of late S or G2 phase of the cell cycle, since cells treated with cycloheximide and blocked in mid-S phase had nascent DNA of a size corresponding to 4-8 joined replicons. Finally, the results support the hypothesis that at least one step in the process of nascent DNA maturation may require replication, during late-S phase, of DNA segments that are interspersed within replicon-clusters that replicate early in the S phase.

DNA and histone synthesis rate change during the S-period in Ehrlich ascites tumor cells

Data from flow-cytometric analysis of DNA of Ehrlich ascites tumor cells were fitted using non-linear least squares curve fitting routines. Analysis of rates of synthesis from the derived S-period profiles revealed a pattern of changing rates of DNA synthesis during the S-period. Three main peaks are seen whose trough to through periods range from 0 to 16%, 16 to 65%, 65 to 100% of the DNA synthesized during S. The differences between the peak rates and rates in the intervening troughs are small, about 10% of the maximum, but these occur reproducibly. Some differences in the DNA distribution profiles, hence rate profiles, can be seen among samples taken at different times during the day. These are thought to reflect the effects of circadian rhythms, but they are not large enough to obscure the general pattern of rate shifts that occur during the S-period. Analyses of radioactivity of 3H-thymidine pulse labelled cells, sorted across the S-period, were in accord with the results obtained from the DNA distributions. A parallel analysis of DNA and histones showed a correspondence in the timing and direction of shifts in rate for both during the middle part of the S-period.

The chemistry of the retina: Function, renewal, rhythms, and the nucleus

Investigation of the chemical activities of retinal cells began with functional chemistry, an origin which can be traced to the pioneering studies of Kühne and Wald. Next to develop was renewal chemistry, which is concerned with the continued replacement of retinal cell structure. A third field of retinal chemistry, more recently evolved, is the analysis of chemical change associated with daily rhythms. In the living cell, all such groups of chemical reactions are merged into a single, integrated metabolism which ultimately is regulated by molecular systems sequestered within the nucleus. Although nuclear chemistry has so far been neglected in vision research, a theory of nuclear metabolism has been maturing in other fields of biological chemistry. One salient aspect of the theory is the temporal stability of the fundamental constituents of the chromosomes: DNA and histone. Alone among all cellular molecules, they may persist indefinitely. Nuclear theory has proved to have important predictive value in preliminary applications to analysis of the mammalian retina. Evidence is presented in the following report which indicates that retinal cells can repair DNA lesions provoked by UV radiation. Other experiments suggest that in retinal nuclei there is a reversible acetylation of histones and non-histone chromosomal proteins-a process which is implicated in gene regulation.

Amphibian lens histones and their relation to the cell cycle

Histones have been electrophoretically separated from acid extracts of the frog lens for the first time. The five conventional histone fractions, representing four electrophoretic bands (f1; f2b, f3; f2a2; and f2a1), are present in both the epithelial and fiber cells. In addition, a fifth fraction was isolated from both sources and the evidence suggests that it may be a tissue-specific histone, possibly related to the lysine-rich f2c fraction found previously only in nucleated erythrocytes. The epithelial cells contain a substantially greater amount of histone than the fiber cells. Moreover, the fibers, unlike the epithelium, manifest no net histone synthesis or turnover following lenticular explantation. Microspectrophotometric, radioautographic, and gel electrophoretic studies indicate that the histones are synthesized in frog lenses concurrently with DNA. Inhibition of DNA synthesis does not completely abolish that of histones but reduces it by about one-half. In the early stages of culture (prior to their synthesis and that of DNA) the histones appear to undergo alterations which are prevented by treatment with cycloheximide.

"Adaptation" and chromosome variability in excised roots of cereals

Heteroploidy, often coinciding with phenotypic or metabolic changes, was found in cereal root cultures. In some cases the changes were apparent after only 4 passages and affected the roots of bread wheat as well as those containing all or some of the wheat and rye genomes. It is unsafe to assume that cultured organised tissues are cytologically stable. The cause of the abnormality is not known but possible mechanisms are discussed.