The G box is a well-characterized cis-acting

We measured the content and metabolism of histone mRNA in mouse 3T6 fibroblasts during a serum-induced transition from the resting to growing state. The content of several histone H3 and H2b mRNAs was measured by an S1 nuclease procedure. All of these increase in parallel by a factor of about 50 during S phase. However, the rate of H3 gene transcription increased only fivefold during this period, as determined in an in vitro transcription assay. This suggests that histone mRNA content is also controlled at the posttranscriptional level. When resting cells were serum stimulated in the presence of cytosine arabinoside, the rate of H3 gene transcription increased to about the same extent as that in control-stimulated cells. However, cytoplasmic H3 mRNA content increased only five to seven-fold. The half-life of H3 mRNA during S phase was about 4 to 5 h. When cytosine arabinoside was added to cells in the S phase, the half-life of the message decreased to about 15 min. The rapid turnover of H3 mRNA was prevented when the drug was added in the presence of cycloheximide or puromycin. The rate of H3 gene transcription decreased by only 35% after treatment with cytosine arabinoside. These results suggest that H3 gene transcription is not tightly coupled to DNA replication but is controlled temporally during the resting to growing transition. However, there is a correlation between the rate of DNA synthesis and the stability of histone H3 mRNA.It has been known for many years that histones are incorporated into chromatin primarily during DNA replication (5,10,12). There are several classes of histone proteins. The replication variants, whose synthesis is correlated with DNA synthesis, are predominant in cultured cells (29). A second class of histone proteins, the replacement variants, differ in only a few amino acids from the replication histone proteins. These are predominant in nondividing cells in differentiated tissue (8), but are also present in rapidly growing cells at low levels (29).Detailed analysis of the biochemical mechanisms responsible for controlling histone gene expression have been facilitated by the molecular cloning of the genes for histones from a variety of organisms. These clones have been used as hybridization probes to study the content and metabolism of the various histone mRNA species and the regulation of histone gene transcription. For example, in HeLa cells and in yeasts, the amount and rate of synthesis of histone mRNA increase greatly between Gl phase and S phase (13,15,23). However, in S49 mouse lymphoma cells selected by centrifugal elutriation, histones are synthesized at a similar rate in both Gl and S phases (11). In exponentially growing cultured cells, the steady-state levels of mRNAs for the replication variants (but not the replacement variants) are rapidly decreased in response to inhibition of DNA synthesis. This is the result of decreases in both the rate of transcription and the stability of the mRNAs (13,25).Cultured mouse 3T6 fibroblasts (26) provide an excellent system for c...

No abstract

We have analyzed the nucleotide sequence and accumulation of an mRNA which is prevalent in seeds of Brassica napus L. During normal development, the mRNA begins to accumulate during late embryogeny, is stored in dry seeds, and becomes undetectable in seedlings within 24 hours after imbibition. Moreover, abscisic acid treatment of embryos precociously induces or enhances accumulation of the mRNA. Nucleotide sequencing studies show that the deduced 30 kDa polypeptide has an unusual primary structure; the polypeptide possesses direct amino acid sequence repeats and is virtually entirely hydrophilic with the exception of a hydrophobic carboxyl-terminal region. Based upon the expression pattern and predicted polypeptide sequence, we conclude that the mRNA is encoded by a late embryogenesis-abundant (Lea) gene in B. napus.

Transcription rates and mRNA levels for Brassica napus seed storage protein families, cruciferin and napin, have been determined in embryos developing in the seed, as well as in embryos cultured with and without abscisic acid. Cruciferin and napin mRNAs are high during the cell expansion phase of embryo development, representing as much as 11 and 8%, respectively, of the total embryo mRNA. During the same time cruciferin and napin gene transcription rates, as measured in isolated nuclei, are also high. The data indicate that cruciferin mRNA is more stable than napin mRNA because while the napin transcription rate is higher than the cruciferin transcription rate, the cruciferin mRNA accumulates to higher levels. However, late in embryo development, both cruciferin and napin mRNAs seem to be less stable than earlier because comparable transcription rates result in lower mRNA levels. When embryos are cultured in the presence of abscisic acid, the levels of cruciferin and napin mRNAs are two-to threefold higher than in embryos cultured on basal medium. The transcription rates show a similar increase in the presence of abscisic acid, suggesting that abscisic acid is responsible for the increased mRNA level at least in part through an increase in the transcription rate of the two genes.Few proteins from plants have been studied as extensively as the seed storage proteins. By definition, seed storage proteins accumulate during the development of the seed, are stable during the period ofdevelopmental arrest that separates embryogeny from germination, and then are specifically degraded The pattern of expression of the storage proteins in B. napus has been followed by measuring protein accumulation (6) and mRNA levels (8,15) in embryos of different developmental stages. The major period of accumulation for both proteins begins during the cell expansion phase, which occurs about half way through seed development (25 dpa3) after tissues and organs have formed and cell division has ceased, and during which storage reserves increase. Napin accumulation stops about 20 d later (45 dpa), when the seeds are drying down (desiccation phase), while cruciferin accumulation continues until just before developmental arrest. Using cDNA clones, the mRNAs for napin and cruciferin can be detected quite early; napin mRNA is detected at 18 dpa just past the 'heart stage' of development and cruciferin mRNA can be detected 5 d later. Napin mRNA levels increase to their maximal level by 33 dpa while cruciferin mRNA levels do not peak until 38 dpa (8,15 Plant Physiol. Vol. 91,1989 seed storage protein mRNAs at rates comparable to development in the seed if 1 to 10 ,M ABA is included in the culture medium. In the absence of ABA, seed storage protein and seed storage protein mRNAs decrease to low levels.Regulation of storage protein expression during development and in culture is likely to be complex. For example, early embryos (27 dpa) cultured on ABA accumulate cruciferin mRNA and protein to levels comparable to that in the seed. Ho...