Isolation and in vitro translation of zein messenger ribonucleic acid

In vitro synthesis of pre-proteins of vacuolar compartmented proteinase inhibitors that accumulate in leaves of wounded tomato plants

Two proteinase inhibitor proteins that are compartmented in leaf vacuoles (lysosomes) were synthesized in vitro. mRNA was isolated from 17-day-old expanding tomato leaves by extraction with chaotropic buffers followed by chromatography on oligo(dT)-cellulose and was translated with a rabbit reticulocyte lysate system. Preparations of mRNA from leaves of both wounded plants and unwounded plants directed the incorporation of equivalent amounts of label into trichloroacetic acid-precipitable proteins. Only mRNA from leaves of wounded plants directed label into proteins that could be immunoprecipitated with rabbit IgG specific for either inhibitor I or inhibitor II. These results indicate that the wound-induced accumulation of proteinase inhibitors I and II in leaf vacuoles is a result of the presence of translatable mRNA species not present in leaves of unwounded plants. Gel electrophoresis of the immunoprecipitates in NaDodSO(4)/urea/polyacrylamide gels revealed that inhibitors I and II were translated in vitro as precursors about 2000 daltons larger than the inhibitors found in leaves. The presence of the additional polypeptide sequences in the newly synthesized inhibitors indicates that the inhibitors are processed either during or after synthesis, and the presequences may be signal peptides that are part of the process of inhibitor transport into the vacuolar compartments of tomato leaf cells.

Messenger RNA for G1 protein of French bean seeds: Cell-free translation and product characterization

The fraction of poly(A)-containing RNA isolated from ripening bean (Phaseolus vulgaris) cotyledons that sedimented at 16 S in linear logarithmic sucrose gradients was at least as active a messenger as viral RNA when added to a cell-free protein-synthesizing system from wheat germ. The major products synthesized in vitro were polypeptides of about 47,000 and 43,000 daltons, corresponding to two of the three subunits of G1 protein, the most abundant bean seed storage protein. No trace of the largest (53,000 daltons) subunit was found among the polypeptides synthesized in vitro. Proof that the 47,000- and 43,000-dalton polypeptides coded by the 16S RNA were indeed subunits of G1 protein was obtained by immunoprecipitation with monovalent antibody to G1 protein and by electrophoretic mapping of peptides on acrylamide gels after digestion of mixtures of authentic protein and radioactive translation products with protease V8, chymotrypsin, and trypsin. The subunits synthesized in vitro were slightly smaller than the native subunits, probably because they lacked the sugar residues present on the holoprotein.

Purification and translation of zein messenger RNA from maize endosperm protein bodies

The messenger RNAs coding for the zein storage protein have been purified from other contaminating RNAs. The average molecular lengths are 1.1-1.2 kilobases, as determined by polyacrylamide gel electrophoresis and by electron microscopy. Products of messenger-dependent protein synthesis in vitro appear to be 1100 and 2000 daltons heavier than the native polypeptides. Zein is like secretory proteins in having a precursor with an additional amino-terminal sequence. Although only one mRNA is seen in polyacrylamide gel electrophoresis, the combined size of the polypeptide products formed exceeds the coding capacity for one message of the size determined in this study. This suggests that there are at least two mRNAs of similar sizes for the zein polypeptides rather than one dicistronic message.

Transposon tagging and molecular analysis of the maize regulatory locus opaque-2

Genetic analyses suggested that the opaque-2 (o2) locus in maize acts as a positive, transacting, transcriptional activator of the zein seed storage-protein genes. Because isolation of the gene is requisite to understanding the molecular details of this regulation, transposon mutagenesis with the transposable element suppressor-mutator (Spm) was carried out, and three mutable o2 alleles were obtained. One of these alleles contained an 8.3-kilobase autonomous Spm, another a 6.8-kilobase nonautonomous Spm, and the third an unidentified transposon that is unrelated to Spm. A DNA sequence flanking the autonomous Spm insertion was verified to be o2-specific and provided a probe to clone a wild-type allele. Northern blots indicated that the gene is expressed in wild-type endosperm but not in leaf tissues or in endosperms homozygous for a mutant allele of the O2 gene. A transcript was detected in endosperms homozygous for mutations at opaque-7 and floury-2, an indication that O2 expression is independent of these two other putative regulators of zein synthesis.

Abscisic acid and the regulation of synthesis of specific seed proteins and their messenger RNAs during culture of soybean embryos

The ability of soybean (Glycine max (L.) Merr.) embryos cultured in vitro in the absence of abscisic acid (ABA) to germinate precociously increased as the embryos matured. Exogenous ABA prevented precocious germination at all stages of development, concentrations below 10(-5) M being partially effective. Growth (fresh weight increase) of mid-maturation embryos was dependent on ABA, and such embryos required ABA for continued synthesis of storage proteins during culture. Two complementary-DNA clones for different members of the family of 11S storage proteins, and one for the 7S storage proteins were used in Northern blot hybridizations to analyze the effects of ABA on the levels of the 11S and 7S mRNAs. In addition, filter hybridizations with in-vivo-labeled [(3)H]polyadenylated RNA to the cloned DNA for one of the 11S proteins were carried out to study transcription of 11S mRNA. Midmaturation embryos cultured with ABA continued to transcribe mRNAs for the storage proteins during the 21 d of culture studied, whereas in the absence of ABA these mRNAs disappeared from the cotyledons within about 5 d. The optimum concentration of ABA for the synthesis of storage-protein mRNAs was 10(-5) M. The effect of ABA on the concentration of 11S mRNAs was stage-dependent. Abscisic acid caused a decrease in the mRNA in embryos cultured at the cotyledon stage; it was necessary for high levels of the mRNA to be achieved in early- and mid-maturation embryos; and it did not reverse the decline in the mRNA levels in embryos cultured at the late-maturation stage.

Synthesis and localization of a development-specific protein in sclerotia of Sclerotinia sclerotiorum

A development-specific protein (SSP) makes up about 35 to 40% of the total protein in sclerotia of the fungus Sclerotinia sclerotiorum. The protein consists of three charge isomers, with one isomer making up 80 to 90% of the total. In vitro translation of poly(A)+ RNA isolated from cells in early stages of sclerotia formation revealed that 44% of the amino acids incorporated was into SSP. In vivo- and in vitro-synthesized forms of SSP migrated at identical rates on both isoelectric focusing and denaturing polyacrylamide gels, indicating that SSP was not synthesized as a larger precursor. This was significant because SSP accumulated in membrane-bound, organellelike structures which resemble protein bodies found in seeds of many higher plants.

Polysome assembly and RNA synthesis during phytochrome-mediated photomorphogenesis in mustard cotyledons

The effect of phytochrome (high irradiance reaction, elicited by continuous far-red light) on cellular polysome levels was investigated using ribosome-isolation procedures which prevent the methodological artifacts inherent in previous studies on polysomes. By including the large pool of ribosomal subunits in the analysis and using the ratio (polysomes: monomers + subunits) as a quantitative estimate of the translational capacity of the ribosomes in mustard (Sinapis alba L.) cotyledons, we found the following results: 1) After a lag-phase of less than 30 min, phytochrome induces a massive increase in the relative amount of cytosolic (free) polysomes at the expense of ribosomal subunits. 2) Cytosolic and membrane-bound polysomes are increased by phytochrome in constant proportions (constant ratio of 65:35 in light and darkness). 3) Simultaneously with the light-mediated increase of the polysome level there is an increased incorporation of newly synthesized (labeled) non-ribosomal RNA, presumably mRNA, into the polysomes which can be kinetically discriminated from the slower incorporation of newly synthesized (labeled) rRNA. 4) Cordycepin strongly inhibits the synthesis of RNA and completely prevents the light-mediated increase of polysomes. 5) The electrophoretic patterns of the in-vitro translation products obtained with polysomal polyadenylated RNA from dark-grown and light-grown cotyledons showed no significant qualitative differences. We conclude from these results that photomorphogenesis of mustard cotyledons is related to a massive increase of newly synthesized mRNA leading to a correspondingly increased recruitment of ribosomal subunits into polysomes. The phytochrome-induced increase of translatable mRNA involves mainly quantitative changes in the production of mRNA species which are also present in the dark-grown cotyledons.

Sequence analysis of zein cDNAs obtained by an efficient mRNA cloning method

A cDNA library was generated from mRNA isolated from the developing endosperm of W22 maize inbred. cDNA clones for zein, the maize storage protein family, were isolated and analyzed by DNA sequencing. The DNA sequences of four clones containing cDNA copies of mRNAs belonging to one zein subfamily were determined. The data support the following conclusions: a) genes encoding the larger of the two zein species contain eleven instead of nine repeat units within the coding sequence of the gene; b) transcription can be terminated at either of the two polyadenlation signals and c) transcription starts 31 basepairs downstream from the first T in the TATA box. To facilitate this analysis a new method for the construction of cDNA libraries was developed. The mRNA was annealed to linearized and oligo-dT tailed pUC9 plasmid DNA, which then primed synthesis of the first strand of the cDNA. Oligo-dG tails were added to the cDNA-plasmid molecules, which were then centrifuged through an alkaline sucrose gradient. The gradient step removed small molecules and separated the two cDNAs which were formerly attached to the same double stranded plasmid molecule. An excess of oligo-dC tailed denatured pUC9 DNA was added and the DNA was renatured under conditions that favor the circularization of monomers by the oligo-dC and oligo-dG tails. The oligo-dC tail served as primer for the synthesis of the second strand of the cDNA. The library was screened by colony hybridization using 32P-labelled cDNA and DNA from genomic zein clones as probes. We obtained 20,000 clones hybridizing total cDNA starting with 1 microgram of plasmid DNA and 1 microgram of mRNA.

In vivo and in vitro synthesis of CM-proteins (A-hordeins) from barley (Hordeum vulgare L.)

CM-proteins from barley endosperm (CMa, CMb, CMc, CMd), which are the main components of the A-hordein fraction, are synthesized most actively 10 to 30 d after anthesis (maximum at 15-20 d). They are synthesized by membranebound polysomes as precursors of higher apparent molecular weight (13,000-21,000) than the mature proteins (12,000-16,000). The largest in vitro product (21,000) is the putative precursor of protein CMd (16,000), as it is selected with anti-CMd monospecific IgG's, and is coded by an mRNA of greater sedimentation coefficient (9 S) than those encoding the other three proteins (7.5 S). CM-proteins always appear in the soluble fraction, following different homogenization and subcellular fractionation procedures, indicating that these proteins are transferred to the soluble fraction after processing.

RNA metabolism and membrane-bound polysomes in relation to globulin biosynthesis in cotyledons of developing field beans (Vicia faba L.)

In cotyledon cells of developing field beans the RNA content per cell does not change in the second half of developmental period 2, whereas globulin biosynthesis continues. The constant RNA content per cell results from an equilibrium between RNA synthesis and degradation. All types of RNA are synthesized until the end of globulin biosynthesis, but poly(A)-containing RNA was preferentially labelled during maximum globulin formation. During stage 2 of seed development of poly(A)-containing RNA fraction represents a discrete peak in the 12--18-S region on agarose gels and corresponds to the peak of poly(A)-containing RNA isolated from polysomes. alpha-Amanitin inhibits selectively the labelling of poly(A)-containing RNA and concomitantly globulin formation. Translation of total poly(A)-containing RNA, free and membrane-bound polysomes in a cell-free wheat germs demonstrates that the globulins are preferentially produced on membrane-bound polysomes and that poly(A)-containing RNA includes the mRNA for both vicilin and legumin.

Cell free synthesis of some storage protein subunits by polyribosomes and RNA isolated from developing seeds of pea (Pisum sativum L.)

Polyribosomes which have template activity in the wheat germ system have been isolated from developing pea seeds. Some of the translation products have identical mobilities to the vicilin and legumin subunits by SDS-PAGE. Certain products were specifically immunoprecipitated with antisera prepared against purified vicilin and legumin fractions. Various RNA fractions including poly A-rich RNA have also been isolated from polyribosomes and shown to direct the synthesis of polyripeptides whose properties are similar to the storage protein subunits. The results are discussed in relationship to other investigations with seed storage protein biosynthesis in vitro.

Electrophoretic fractionation and translation in vitro of poly(rA)-containing RNA from maize endosperm. Evidence of two mRNAs coding for zein protein

In the search for individual mRNAs coding for particular zein proteins, polysomal RNA was isolated from the endosperm of 22-days post-pollination maize kernels. This RNA was enriched for poly(rA)-containing RNAs and then submitted to preparative polyacrylamide gel electrophoresis under denaturing conditions. After electrophoresis, RNA fractions were eluted from the polyacrylamide gel and analyzed for zein mRNA activity by translation in vitro in the wheat germ system. The wheat germ system had previously been optimized for accurate translation of zein mRNAs. By a gel-electrophoretic analysis of immunoprecipitated products from the translation reactions in vitro, it could conclusively be shown that the endosperm of developing maize kernels contains two separable mRNAs for zein, one coding for the 22000-Mr protein and one coding for the 19000-Mr protein.

Preparation of a complementary DNA for leghaemoglobin and direct demonstration that leghaemoglobin is encoded by the soybean genome

In soybean root nodules, leghaemoglobin (Lb) accounts for 25--30% of the total soluble protein but is not detected in other tissues. In order to determine whether the Lb genes are plant or bacterial in origin a cDNA probe for Lb was prepared from 9S poly (A) containing mRNA of root nodules. Although this 9S mRNA directed synthesis of predominantly three forms of Lb in vitro, the kinetics of hybridisation of cDNA and the 9S mRNA showed a transition at about 30% hybridisation which suggested that the 9S-cDNA was not pure Lb-cDNA. The abundant, Lb-cDNA was prepared by two cycles of hybridising 9S mRNA and cDNA to a Rot of 3 X 10(-3) and isolation of the hybridised cDNA on hydroxyapatite. The Lb-cDNA was homogeneous in hybridisation analysis with 9S mRNA and electrophoresis in 98% formamide gels. This cDNA hybridised with soybean DNA and not with Rhizobium DNA, thus directly demonstrating that Lb genes are of plant origin. Titration of Lb-cDNA with soybean DNA showed that Lb genes are reiterated about forty-fold per haploid genome.

In vitro translation of nematode cuticular collagens

Phenanthroline treatment of growing cultures of the free-living nematode Panagrellus silusiae was used to lower the degree of hydroxylation of nascent collagen chains at the polysomal level. Under these conditions, the bound pentasome-hexasome fraction provided substrate for prolyl hydroxylase. When this polysomal fraction was subsequently tested in a cell-free wheat germ system, collagenase-susceptible translation products were observed after sodium dodecyl sulfate-acrylamide gel electrophoresis. The electrophoretic mobilities of each of these four major collagen products were similar to four collagens that are isolated from intact cuticles. In addition, purified polysomal RNA that adhered to unmodified cellulose directed the synthesis of four pepsin-resistant polypeptides that had molecular weights that coincided with four pepsin-resistant collagens that can be purified from the cuticle of this species. Thus, the polysomal site of the messenger RNAs for the cuticular collagens of P. silusiae was located. Although precursor forms of the cuticular collagens were not produced in the cell-free system, the question whether additional amino acid segments occur on the primary translational products of the cuticular collagens in vivo remains open.

In vitro translation of polyadenylate-containing RNAs from dormant and germinating spores of the fungus Botryodiplodia theobromae

Polyadenylated RNA isolated by oligodeoxythymidylate-cellulose chromatography from spores of the fungus Botryodiplodia theobromae was translated in a cell-free protein-synthesizing system derived from wheat embryo. Reaction conditions which would yield efficient and accurate in vitro translation of the spore RNA were established. Dual isotopically labeled mixtures of in vitro translation products from germinated and dormant spore polyadenylated RNA, as well as polyadenylated RNAs from intermediate stages of germination, produced qualitatively similar gel electrophoresis patterns, with polypeptides of 10,000 to 55,000 molecular weight. Proteins synthesized in vivo and extracted from germinating spores at three different stages possessed a greater size range, with molecular weights up to 85,000, although the in vitro synthesis apparently did yield the lower-molecular-weight proteins which were synthesized in vivo. Tryptic digest patterns of proteins translated in vitro from polyadenylated RNA of dormant and germinated spores were found to be identical in positions in only 40% of the spots. Furthermore, a dual-label comparison by isoelectric focusing of proteins translated from polyadenylated RNA of germinated and dormant spores also showed qualitative and quantitative differences among the in vitro translation products. We conclude that there are differences between the in vitro translation products of mRNA from dormant and germinated spores and that the mRNA preserved in the dormant spores contains genetic information which is qualitatively different from that of the germinated spores.

Effects of floury-2 locus on zein accumulation and RNA metabolism during maize endosperm development

Zein accumulation patterns during mutant and normal maize endosperm development were determined. Accompanying an increase in the number of floury-2 alleles present in the endosperm was a well-defined stepwise depression in zein accumulation. Analysis of the zein accumulated in endosperms containing zero, one, two, and three doses of the floury-2 allele by sodium dodecylsulfate--polyacrylamide gel electrophoresis revealed a proportionate reduction in the two major zein components, Z1 and Z2. In contrast, the relative proportions of the minor zein bands were altered. Membrane-bound polysomes isolated from kernels of floury-2 and normal maize were predominantly large size classes. The presence of increasing numbers of the floury-2 allele in the endosperm decreased recovery of membrane-bound polysomal material in a stepwise fashion. However, major alterations in polysome size-class distributions were not observed. The reduction in membrane-bound polysome material correlated linearly with reductions in in vitro zein synthesis and in vivo zein accumulation.