The time of replication of the two H4 histone genes (H41 and H42) was determined during the naturally synchronous mitotic cycle of Physarum polycephalum. 5-Bromo-2'-deoxyuridine labeling and density gradient centrifugation was used to isolate newly synthesized DNA from defined periods of S phase. The DNA was analyzed by Southern hybridization with a cloned probe containing one of the H4 histone genes ofPhysarum. The results indicate that the two H4 histone genes are replicated in the first 30 min of S phase but not exactly at the same time. H41 is replicated during the frt 10 min of S phase, when only 15% of the genome is duplicated, whereas H42 replicates between 20 and 30 min after the onset of S phase. The possible relationship between the periodic expression of the genes and the timing of their replication is discussed.In eukaryotes, as in prokaryotes, the replication of specific DNA sequences follows a distinct temporal order (1, 2). DNA replicated in one period of the S phase is replicated in the same temporal interval of the next S phase. In early experiments autoradiography and cytology was used to demonstrate that the chronological order of replication of specific DNA segments is invariant from one cell generation to the next (3).In mammalian cells, multiple copy sequences have been found to replicate mainly late in S phase, whereas families of middle-repetitive sequences replicate either early or late (for references see ref. 4). "Housekeeping" or active tissuespecific genes are generally replicated early. Furst et al. (5) and Epner et al. (6) have, for example, demonstrated that the replication of globin genes in mouse erythroleukemia cells is predominantly restricted to the first third of S phase. In the same manner, studies by Braunstein et al. (7) have shown that the replication of the immunoglobulin heavy chain constant region gene segments CQ, Cy, and C, is restricted to the first half of S and follows the linear order in which they are arranged in the genome. Recently Calza et al. (8) have presented evidence that certain low-copy-number genes can be replicated late and have suggested that the gene's position in the chromosome is important in determining the time during S at which it is replicated. All the results presented on the timing of gene replication support the idea that earlyreplicating genes are capable of expression, and Goldman et al. (4) suggest that the switching of specific genes from an early to a late replication region reflects the commitment of that gene to quiescence.The sustained synchrony over several successive nuclear division cycles in the syncytial plasmodia phase of the myxomycete Physarum polycephalum was exploited by Braun et al. (9) to demonstrate that chromosomal DNA is replicated in a defined temporal sequence. In addition, Fouquet and Sauer (10) have shown that repetitive DNA of Physarum is synthesized during late S phase. At the level of specific genes, Pierron et al. (11) have recently found that the four actin loci are replicated in an invariant tempo...
The myxomycete Physarum polycephalum contains two types of H4 histone genes. Southern blotting of restriction endonuclease fragments of P. polycephalum DNA and hybridization to a cloned probe labelled by nick‐translation indicate that there are only one or two copies of each H4 gene per haploid genome. A cloned homologous genomic probe was used to study the cellular abundance of H4 mRNA during the cell cycle. We report that the H4 mRNA is not only transcribed in S phase as previously described for other organisms but that transcription of the H4 gene also occurs at the end of G2 phase. Since no translation of the histone messenger was observed in G2 phase this suggests that the histone mRNA synthesized in G2 constitutes a pool of molecules in anticipation of the next S phase.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.