Absence of poly (A) in a large part of newly synthesized casein mRNAs

Translational regulation of milk protein synthesis at secretory activation

Studies conducted since the 1970s have revealed that the production of milk proteins in the mammary gland under the influence of lactogenic hormones (insulin, prolactin, and glucocorticoids) is regulated at multiple levels. Whereas earlier studies concentrated on transcriptional regulation and stabilization of milk protein mRNAs, more recent studies have revealed that translation of milk protein mRNAs is also dependent on lactogenic hormones. A general stimulation of translation in mammary epithelial cells is caused by amino acids (as signaling molecules) or by phosphorylation of the translational regulator 4E-BP1 in a synergistic response to signals from insulin and prolactin. However, a selective enhancement of milk protein mRNA translation is caused by cytoplasmic polyadenylation of mRNA, again in a synergistic response to these two hormones. Preliminary evidence indicates that the latter effect depends on the existence of a cytoplasmic polyadenylation element (CPE) in milk protein mRNAs and phosphorylation of its binding protein, CPEB. Experiments in whole animals, organ explants, and cell culture have shown that the poly(A) length of milk protein mRNAs changes as a function of the lactation cycle. Interestingly, cytoplasmic polyadenylation is likely to be responsible for the selective hormone-dependent enhancement of both translation and stability of milk protein mRNAs.

Full-length cDNAs: more than just reaching the ends

The development of functional genomic resources is essential to understand and utilize information generated from genome sequencing projects. Central to the development of this technology is the creation of high-quality cDNA resources and improved technologies for analyzing coding and noncoding mRNA sequences. The isolation and mapping of cDNAs is an entrée to characterizing the information that is of significant biological relevance in the genome of an organism. However, a bottleneck is often encountered when attempting to bring to full-length (or at least full-coding) a number of incomplete cDNAs in parallel, since this involves the nonsystematic, time consuming, and labor-intensive iterative screening of a number of cDNA libraries of variable quality and/or directed strategies to process individual clones (e.g., 5' rapid amplification of cDNA ends). Here, we review the current state of the art in cDNA library generation, as well as present an analysis of the different steps involved in cDNA library generation.

Cloning of non-polyadenylated RNAs from rat brain

Rodent brain has been reported to contain a fraction of non-polyadenylated (poly(A)-) mRNA that includes about 100,000 different sequences, most of which are not found in the poly(A)+ fraction. We have prepared a cDNA library of low-abundance poly(A)- RNAs from rat brain polysomes, and have characterized three clones in detail. Two of the clones hybridize on Northern blots to poly(A)+ RNAs from brain. Dot blot hybridization and RNase protection assays demonstrate that although the bulk of the RNA complementary to these clones is present in the poly(A)- fraction, a small portion (7-21%) is present in the poly(A)+ fraction. Our results suggest that the poly(A)-mRNA fraction from rat brain may not contain sequences that are different from those in the poly(A)+ fraction.

Variation in the lack of polyadenylation of the rat milk protein mRNAs during the lactation cycle

Translationally active milk protein mRNAs were found as nonpolyadenylated mRNAs in the rat mammary gland during pregnancy, lactation and involution. Analyses of whey protein mRNA and casein mRNA with the corresponding cDNAs showed that the lack of polyadenylation of these mRNAs at different time points of the lactation cycle is not consistent with the hypothesis that polyadenylation may be incomplete in the mammary gland when large amounts of mRNA are synthesized. The fraction of whey protein mRNA and casein mRNA that lacked polyadenylation was inversely proportional to the concentration of each sequence in the tissue during pregnancy, lactation and involution. A model is proposed to explain the finding that in each animal the ratio of casein mRNA to whey protein mRNA was similar in polyadenylated RNA and in nonpolyadenylated RNA at all stages of the lactation cycle.

Variation of the poly(A) size classes in the rat mammary gland during the lactation cycle

The sizes of the poly(A) tracts associated with rat mammary RNA were determined at several time points in the lactation cycle. The poly(A) tracts in the lactating gland displayed two predominant size class peaks at 80-85 and 45-47 residues. The 9S whey protein mRNA and the 15S casein mRNA purified from the 12 day lactating mammary gland both contained poly(A) tracts displaying a similar size distribution. The 45 residue tracts were a characteristic of lactation; they were not found at 8 days of pregnancy and only small amounts of these shorter poly(A) tracts were found in the 16 day pregnant gland. The poly(A) tracts of the involuted gland displayed the same size characteristics as those of late pregnancy. At all the developmental stages that were examined, the fraction of 45 residue poly(A) tracts was always proportional to the total poly(A) content of the mammary cells.

Effect of 7-methylguanosine-5'-phosphate on the translation of rat milk-protein mRNAs in the wheat-germ cell-free system

The translation of polyadenylated and of non-polyadenylated RNA obtained from lactating rat mammary gland was almost totally inhibited by 0.5 mM 7-methylguanosine-5'-phosphate in the wheat-germ cell-free system. This inhibition was maintained during the preparation of the 9S whey-protein mRNA and of the 12S and 15S casein mRNAs. Chemical decapping of these mRNAs caused a similar reduction of their activity. Although a large fraction of milk-protein mRNAs have been reported to lack 3'-polyadenylation, these results show that the mRNAs in the mammary gland do contain a 5'-terminal 7-methylguanosine cap.

Cytoplasmic polyadenylate processing events accompany the transfer of mRNA from the free mRNP particles to the polysomes in Physarum

The relationship between the mRNA in the polysomes and the free cytoplasmic messenger ribonucleoprotein of Physarum polycephalum was studied by microinjection techniques. Labeled free cytoplasmic ribonucleoprotein, prepared from donor plasmodia, was microinjected into unlabeled host plasmodia, and its fat was followed in the host ribonucleoprotein particles. Approximately one-half of the poly(A)-containing RNA [poly(A)+RNA] that originated from the microinjected particles was incorporated into the host polysomes by normal translational processes within 1 hr. Very short poly(A) sequences (approximately 15 nucleotide residues) were found in these poly(A)+RNA molecules. These short poly(A) sequences were sensitive to digestion with micrococcal nuclease, suggesting that they were not associated with protein. Because the poly(A)+RNA molecules of the microinjected free cytoplasmic mRNP had originally contained poly(A) sequences 50-65 nucleotides long and were associated with protein extensive poly(A) degradation and poly(A).protein complex dissociation must have occurred during their incorporation into the polysomes or during their translation. These results demonstrate a precursor-product relationship between free cytoplasmic mRNP and polysomal mRNA and suggest that the incorporation process in Physarum is accompanied by structural modifications in the poly(A) region of mRNA. They also imply that the polysome is a site for disruption of the poly(A).protein complex and poly(A) degradation.

Relative distribution of post-nuclear poly(A)-containing RNA abundance groups within the nuclear and post-nuclear polyadenylated and non-polyadenylated RNA populations of the lactating guinea-pig mammary gland

1. RNA isolated from the post-nuclear supernatant of the lactating guinea-pig mammary gland was fractionated with oligo(dT)-cellulose into three populations; those that bound at ;low salt' [long poly(A) tracts, 78-32 nucleotides]; those that bound at ;high salt' [shorter poly(A) tracts, 48-21 nucleotides]; and those that did not bind [no poly(A) or short poly(A) tracts, <20 nucleotides]. Nuclear RNA was fractionated into two populations, those that bound in ;low salt' and those that did not bind. All the post-nuclear RNA fractions directed the synthesis of milk proteins in a Krebs II ascites cell-free system. 2. (3)H-labelled DNA complementary to the post-nuclear poly-(A)-containing RNA population (low-salt fraction) was fractionated into abundant (milk-protein mRNA), moderately abundant and scarce sequences. This complementary DNA was then used to investigate the distribution of the mRNA sequences in the different RNA populations. This showed that all sequences were present in polyadenylated and non-polyadenylated fractions, but that major quantitative differences were apparent. The abundant milk-protein mRNA sequences predominated in the ;low-salt' post-nuclear poly(A)-containing RNA fraction, whereas the moderately abundant sequences predominated in the non-polyadenylated post-nuclear RNA fraction. In total cellular RNA, those sequences deemed initially to be moderately abundant within the ;low-salt' poly(A)-containing RNA population were present at a concentration very similar to those of the abundant milk-protein mRNA (approx. 6x10(5) copies of each sequence/cell). Similarly, analysis of the nuclear RNA populations showed that the ;abundant' and so-called ;moderately abundant' sequences were present in essentially identical concentrations (2x10(3) copies of each sequence/cell). The majority of these (90-95%) were non-polyadenylated. 3. The results are discussed in terms of the post-transcriptional mechanisms involved in the regulation of gene expression in the lactating guinea-pig mammary gland.

Role of prolactin in the transcription of beta-casein and 28-S ribosomal genes in the rabbit mammary gland

Isolated mammary nuclei were incubated in the presence of HgCTP and the neosynthesized RNA was isolated with a SH-Sepharose column. The concentration of beta-casein mRNA and 28-S ribosomal RNA in the neosynthesized RNA fractions was evaluated using [3H]DNA probes complementary to beta-casein mRNA and 28-S rRNA respectively. In the unstimulated pseudopregnant rabbit, the transcription of both genes was easily detectable. Injections of prolactin progressively enhanced the transcription rate of both genes and preferentially the beta-casein gene. A comparison between the transcription rates and the accumulation of the corresponding gene products in the cell revealed that there is a good correlation between these two parameters for the 28-S rRNA gene. By contrast, the acceleration of beta-casein gene transcription by prolactin is unable to account for the simultaneous accumulation of beta-casein mRNA, indicating that prolactin is a potent stabilizer of casein mRNA. Injections of CB154 into lactating rabbits (a drug which suppresses the secretion of prolactin by hypophysis), induced a rapid drop of beta-casein mRNA concentration and a slow decline of beta-casein gene transcription. Simultaneously the drug was responsible for a marked and rapid decrease of 28-S rRNA gene transcription, while the concentration of the rRNA remained elevated. During weaning the transcription of the beta-casein gene and, to a lower degree, the transcripton of the 28-S rRNA gene proceeded more slowly and this phenomenon was accompanied by a progressive decline of the RNA concentrations.

Polyadenylate-deficient analogues of poly(A)-containing mRNA sequences in cultured AKR mouse embryo cells

Five to six percent (by mass) of AKR-2B mouse embryo cell polysomal RNA consists of messenger RNA sequences which may exist in polyadenylated form. In the steady state, however, only 30--40% of these molecules are retained by extensive passage over oligo(dT)-cellulose, the remainder being present in the form of poly(A)-deficient analogues. Within experimental limits, these poly(A)-deficient analogues contain representatives of all poly(A)-containing mRNA sequences in these cells. An analysis of the kinetics of hybridization of cDNA probes enriched for either abundant or rare poly(A)-containing mRNA sequences suggests that the frequency distributions of poly(A)-containing and poly(A)-deficient analogues are dissimilar, and that a relationship exists between the intracellular frequency of a given mRNA sequence and the number of poly(A)-deficient analogues of that sequence. High frequency sequences appear to be enriched in the poly(A)-containing fraction, while low frequency sequences are predominately associated with the poly(A)-deficient fraction, thus, poly(A) may play a role in the regulation of mRNA frequency in the cytoplasm.

Complex population of nonpolyadenylated messenger RNA in mouse brain

The complexity of nonadenylated mRNA [poly(A)-mRNA] has been determined by hybridization with single-copy DNA (scDNA) and cDNA. Our results show that poly(A)- and poly(A)+ mRNA are essentially nonoverlapping (nonhomologous) sequence populations of similar complexity. The sum of the complexities of poly(A)+ mRNA and poly(A)- mRNA is equal to that of total polysomal RNA or total mRNA, or the equivalent of approximately 1.7 x 10(5) different sequences 1.5 kb in length. Poly(A)- mRNA, isolated from polysomal RNA by benzoylated cellulose chromatography, hybridized with 3.6% of the scDNA, corresponding to a complexity of 7.8 x 10(4) different 1.5 kb sequences. The equivalent of only one adenosine tract of approximately 20 nucleotides per 100 poly(A)- mRNA molecules 1.5 kb in size was observed by hybridization with poly(U). cDNA was transcribed from poly(A)- mRNA using random oligonucleotides as primers. Only 1-2% of the single-copy fraction of this cDNA was hybridized using poly(A)+ mRNA as a driver. These results show that poly(A)- mRNA shares few sequences with poly(A)+ mRNA and thus constitutes a separate, complex class of messenger RNA. These measurements preclude the presence of a complex class of bimorphic mRNAs [that is, species present in both poly(A)+ and poly(A)- forms] in brain polysomes.

Isolation and characterization of trout testis protamine mRNAs lacking poly (A)

Poly(A)+ protamine mRNA was isolated from trout testis cells in a very pure form, and artificial poly(A)- protamine mRNA molecules were derived from it by enzymatic deadenylation with RNAase H from calf thymus after hybridization with oligo(dT). The deadenylated protamine mRNA was found to be active in a wheat germ cell-free system and yielded a labeled product which co-migrated with authentic protamine. These deadenylated mRNA molecules were subsequently used as markers on denaturing polyacrylamide gels to identify and allow the purification of the poly(A)- protamine components known to exist in vivo in the total cellular poly(A)- RNA. RNA species of molecular weights similar to the enzymatically deadenylated subcomponents of protamine mRNA were observed in the natural poly(A)-RNA population of the testis cells. These naturally occurring poly(A)- protamine mRNAs were isolated by preparative gel electrophoresis and further characterized by 3H-poly(U) hybridization assay, by hybridization to complementary DNA made against highly purified poly(A)+ protamine mRNA, and by their ability to direct protamine synthesis in a cell-free system.

The distribution of poly(A)+ and poly(A)- protamine messenger RNA sequences in the developing trout testis

Protamine messenger RNA was isolated in a very pure form from trout testes and used as a template for the synthesis of labeled complementary DNA (cDNA) of high specifiv activity. The cDNA was found to be a full-length transcript of protamine messenger RNA and was used as a probe for hybridization reactions with RNA preparations isolated from three subcellular compartments of differentiating trout testis cells. The RNA populations from the nuclei, polysomes, and postribosomal supernatant of these cells were fractionated into poly(A)-containing [poly(A)+] and poly(A)-free [poly(A)-] RNA to determine the distribution of these two forms of protamine mRNA in these cell compartments. At the early protamine stage of testis development, polysomal and postribosomal supernatant fractions contain almost equal quantities of poly(A)+ protamine mRNA, but poly(A)- protamine mRNA was found almost entirely in the polysomes.