The cloning, genetic mapping, and expression of the constitutive sucrose synthase locus of maize

Two differentially expressed genes encode isoenzymes of sucrose synthase in Zea mays. A clone of the shrunken 1 (Sh1) locus, the structural gene for the major endosperm form of sucrose synthase, was used to isolate a genomic clone of constitutive sucrose synthase (Css), the structural gene for the isoenzyme expressed in embryo and other tissues. The Css clone was positively identified by RNA blot analysis of RNA from wild type and a sh1 deletion stock and by analysis of the in vitro translation product of hybrid-selected mRNA. Southern blot analysis of DNA from monosomic plants derived from an r-x1 stock, coupled with restriction fragment length polymorphism mapping, placed the Css gene 32 map units from Sh1 on chromosome 9. In seedling tissues, Css mRNA is present at higher levels than Sh1 mRNA. Expression of both Sh1 and Css in root tissue is enhanced by anaerobic conditions, although Css is induced to a lesser extent than is Sh1. Thus, Css appears to be expressed constitutively, whereas Sh1 is expressed at high levels only in response to specific developmental and environmental stimuli.

Multiple forms of maize endosperm adp-glucose pyrophosphorylase and their control by shrunken-2 and brittle-2

Heat-labile and heat stable forms of ADP-glucose pyrophosphorylase were identified in the maize endosperm. The heat-labile form is destroyed by normal electrophoretic conditions. The heat-stable form corresponds to pyrophosphorylase B. In wild type, 96% of the total activity is heat labile. Both forms are reduced in 11 brittle-2 (bt2) and 12 shrunken-2 (sh2) mutants. The heat-labile form is reduced to a greater extent than is the heat-stable form in each of the 23 mutants. Deletion of sh2 abolishes both forms. The original ratio of the two forms is restored after sh2 function is expressed via transposition of Dissociation from sh2. The possible roles of these genes in the control of ADP-glucose pyrophosphorylase are discussed.

Maize genes encoding the small subunit of ADP-glucose pyrophosphorylase

Plant ADP-glucose pyrophosphorylase (AGP) is a heterotetrameric enzyme composed of two large and two small subunits. Here, we report the structures of the maize (Zea mays) genes encoding AGP small subunits of leaf and endosperm. Excluding exon 1, protein-encoding sequences of the two genes are nearly identical. Exon 1 coding sequences, however, possess no similarity. Introns are placed in identical positions and exhibit obvious sequence similarity. Size differences are primarily due to insertions and duplications, hallmarks of transposable element visitation. Comparison of the maize genes with other plant AGP small subunit genes leads to a number of noteworthy inferences concerning the evolution of these genes. The small subunit gene can be divided into two modules. One module, encompassing all coding information except that derived from exon 1, displays striking similarity among all genes. It is surprising that members from eudicots form one group, whereas those from cereals form a second group. This implies that the duplications giving rise to family members occurred at least twice and after the separation of eudicots and monocot cereals. One intron within this module may have had a transposon origin. A different evolutionary history is suggested for exon 1. These sequences define three distinct groups, two of which come from cereal seeds. This distinction likely has functional significance because cereal endosperm AGPs are cytosolic, whereas all other forms appear to be plastid localized. Finally, whereas barley (Hordeum vulgare) reportedly employs only one gene to encode the small subunit of the seed and leaf, maize utilizes the two genes described here.

rgf1, a mutation reducing grain filling in maize through effects on basal endosperm and pedicel development

The maize cob presents an excellent opportunity to screen visually for mutations affecting assimilate partitioning in the developing kernel. We have identified a defective kernel mutant termed rgf1, reduced grain filling, with a final grain weight 30% of the wild type. In contrast with most defective endosperm mutants, rgf1 shows gene dosage-dependent expression in the endosperm. rgf1 kernels possess a small endosperm incompletely filling the papery pericarp, but embryo development is unaffected and the seeds are viable. The mutation conditions defective pedicel development and greatly reduces expression of endosperm transfer layer-specific markers. rgf1 exhibits striking morphological similarities to the mn1 mutant, but maps to a locus approximately 4 cM away from mn1 on chromosome 2 of maize. Despite reduced starch accumulation in the mutant, no obvious lesion in starch biosynthesis has been detected. Free sugar levels are unaltered in rgf1 endosperm. Rates of sugar uptake, measured over short (8 h) periods in cultured kernels, are increased in rgf1 compared to the wild type. rgf1 and wild-type kernels, excised at 5 DAP and cultured in vitro also develop differently in response to variations in sugar regime: glucose concentrations above 1% arrest placentochalazal development of rgf1 kernels, but have no effect on cultured wild-type kernels. These findings suggest that either uptake or perception of sugar(s) in endosperm cells at 5-10 DAP determines the rgf1 kernel phenotype.

A splice site mutant of maize activates cryptic splice sites, elicits intron inclusion and exon exclusion, and permits branch point elucidation

DNA sequence analysis of the bt2-7503 mutant allele of the maize brittle-2 gene revealed a point mutation in the 5' terminal sequence of intron 3 changing GT to AT. This lesion completely abolishes use of this splice site, activates two cryptic splice sites, and alters the splicing pattern from extant splice sites. One activated donor site, located nine nt 5' to the normal splice donor site, begins with the dinucleotide GC. While non-consensus, this sequence still permits both trans-esterification reactions of pre-mRNA splicing. A second cryptic site located 23 nt 5' to the normal splice site and beginning with GA, undergoes the first trans-esterification reaction leading to lariat formation, but lacks the ability to participate in the second reaction. Accumulation of this splicing intermediate and use of an innovative reverse transcriptase-polymerase chain reaction technique (J. Vogel, R.H. Wolfgang, T. Borner [1997] Nucleic Acids Res 25: 2030-2031) led to the identification of 3' intron sequences needed for lariat formation. In most splicing reactions, neither cryptic site is recognized. Most mature transcripts include intron 3, while the second most frequent class lacks exon 3. Traditionally, the former class of transcripts is taken as evidence for the intron definition of splicing, while the latter class has given credence to the exon definition of splicing.

Maize transposable element Ds is differentially spliced from primary transcripts in endosperm and suspension cells

The process by which transposable elements are spliced from the host gene transcripts remains poorly understood. We previously reported that a maize transposable element Ds (dissociation) and a copy of its host site duplication are perfectly spliced from the shrunken-2 transcript in the endosperm. Here, we have monitored splicing of the Ds element and its flanking Sh2 sequence following transient expression in maize suspension cells. The pattern of Ds splicing in suspension cells differs dramatically from that in the endosperm. In contrast to splicing in the endosperm, Ds in suspension cells was completely spliced from the transcripts using multiple donor and acceptor splice sites outside the element. In addition, noncanonical splice sites were utilized in suspension cells. Our results indicate that this difference in splicing is due to the context of Ds placement in the construct and/or to tissue specific differences in splicing.

Enhanced stability of maize endosperm ADP-glucose pyrophosphorylase is gained through mutants that alter subunit interactions

Temperature lability of ADP-glucose pyrophosphorylase (AGP; glucose-1-phosphate adenylyltransferase; ADP: alpha-D-glucose-1-phosphate adenylyltransferase, EC 2.7.7.27), a key starch biosynthetic enzyme, may play a significant role in the heat-induced loss in maize seed weight and yield. Here we report the isolation and characterization of heat-stable variants of maize endosperm AGP. Escherichia coli cells expressing wild type (WT) Shrunken2 (Sh2), and Brittle2 (Bt2) exhibit a reduced capacity to produce glycogen when grown at 42 degreesC. Mutagenesis of Sh2 and coexpression with WT Bt2 led to the isolation of multiple mutants capable of synthesizing copious amounts of glycogen at this temperature. An increase in AGP stability was found in each of four mutants examined. Initial characterization revealed that the BT2 protein was elevated in two of these mutants. Yeast two-hybrid studies were conducted to determine whether the mutant SH2 proteins more efficiently recruit the BT2 subunit into tetramer assembly. These experiments showed that replacement of WT SH2 with the heat-stable SH2HS33 enhanced interaction between the SH2 and BT2 subunits. In agreement, density gradient centrifugation of heated and nonheated extracts from WT and one of the mutants, Sh2hs33, identified a greater propensity for heterotetramer dissociation in WT AGP. Sequencing of Sh2hs33 and several other mutants identified a His-to-Tyr mutation at amino acid position 333. Hence, a single point mutation in Sh2 can increase the stability of maize endosperm AGP through enhanced subunit interactions.

Maize endosperm ADP-glucose pyrophosphorylase SHRUNKEN2 and BRITTLE2 subunit interactions

ADP-glucose pyrophosphorylase (AGP) represents a key regulatory step in polysaccharide synthesis in organisms ranging from bacteria to plants. Higher plant AGPs are complex in nature and are heterotetramers consisting of two similar but distinct subunits. How the subunits are assembled into enzymatically active polymers is not yet understood. Here, we address this issue by using naturally occurring null mutants of the Shrunken2 (Sh2) and Brittle2 (Bt2) loci of maize as well as the yeast two-hybrid expression system. In the absence of the maize endosperm large AGP subunit (SH2), the BT2 subunit remains as a monomer in the developing endosperm. In contrast, the SH2 protein, in the absence of BT2, is found in a complex of 100 kD. A direct interaction between SH2 and BT2 was proven when they were both expressed in yeast. Several motifs are essential for SH2:BT2 interaction because truncations removing the N or C terminus of either subunit eliminate SH2:BT2 interactions. Analysis of subunit interaction mutants (sim) also identified motifs essential for protein interactions.

Phosphorylation of serine-15 of maize leaf sucrose synthase. Occurrence in vivo and possible regulatory significance

Experiments were conducted to determine whether sucrose synthase (SuSy) was phosphorylated in the elongation zone of maize (Zea mays L.) leaves. The approximately 90-kD subunit of SuSy was 32P-labeled on seryl residue(s) when excised shoots were fed [32P]orthophosphate. Both isoforms of SuSy (the SS1 and SS2 proteins) were phosphorylated in vivo, and tryptic peptide-mapping analysis suggested a single, similar phosphorylation site in both proteins. A combination of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and automated Edman sequencing analysis unequivocally identified the phosphorylation site in the maize SS2 protein as serine-15. This site was phosphorylated in vitro by endogenous protein kinase(s) in a strictly Ca(2+)-dependent manner. A synthetic peptide, based on the phosphorylation site sequence, was used to identify and partially purify an endogenous Ca(2+)-dependent protein kinase(s) from the maize leaf elongation zone and expanding spinach leaves. Phosphorylation of SuSy in vitro selectively activates the cleavage reaction by increasing the apparent affinity of the enzyme for sucrose and UDP, suggesting that phosphorylation may be of regulatory significance. Conservation of the phosphorylation site, and the sequences surrounding it, among plant species suggests that phosphorylation of SuSy may be widespread, if not universal, in plants.

A single mutation that increases maize seed weight

The maize endosperm-specific gene shrunken2 (Sh2) encodes the large subunit of the heterotetrameric starch synthetic enzyme adenosine diphosphoglucose pyrophosphorylase (AGP; EC 2.7.7.27). Here we exploit an in vivo, site-specific mutagenesis system to create short insertion mutations in a region of the gene known to be involved in the allosteric regulation of AGP. The site-specific mutagen is the transposable element dissociation (Ds). Approximately one-third (8 of 23) of the germinal revertants sequenced restored the wild-type sequence, whereas the remaining revertants contained insertions of 3 or 6 bp. All revertants retained the original reading frame 3' to the insertion site and involved the addition of tyrosine and/or serine. Each insertion revertant reduced total AGP activity and the amount of the SH2 protein. The revertant containing additional tyrosine and serine residues increased seed weight 11-18% without increasing or decreasing the percentage of starch. Other insertion revertants lacking an additional serine reduced seed weight. Reduced sensitivity to phosphate, a long-known inhibitor of AGP, was found in the high seed-weight revertant. This alteration is likely universally important since insertion of tyrosine and serine in the potato large subunit of AGP at the comparable position and expression in Escherichia coli also led to a phosphate-insensitive enzyme. These results show that single gene mutations giving rise to increased seed weight, and therefore perhaps yield, are clearly possible in a plant with a long history of intensive and successful breeding efforts.

De novo synthesis of an intron by the maize transposable element Dissociation

The mechanisms by which introns are gained or lost in the evolution of eukaryotic genes remain poorly understood. The discovery that transposable elements sometimes alter RNA splicing to allow partial or imperfect removal of the element from the primary transcripts suggests that transposons are a potential and continuing source of new introns. To date, splicing events that precisely restore the wild-type RNA sequence at the site of insertion have not been detected. Here we describe alternative RNA splicing patterns that result in precise removal of a Dissociation (Ds) insertion and one copy of its eight-nucleotide host site duplication from an exon sequence of the maize shrunken2-mutabe1 (sh2-m1) mutant. In one case, perfect splicing of Ds was associated with aberrant splicing of an intron located 32 bp upstream of the insertion site. The second transcript type was indistinguishable from wild-type mRNA, indicating that Ds was spliced like a normal intron in about 2% of the sh2-m1 transcripts. Our results suggest that the transposition of Ds into sh2 in 1968, in effect, marked the creation of a new intron in a modern eukaryotic gene. The possibility of precise intron formation by a transposable element demonstrated here may be a general phenomenon of intron formation, since consensus intron splice sites can be explained by insertions that duplicate host sequences upon integration. A model is presented.

Structural features of the maize sus1 gene and protein

Genomic clones, cDNA clones, and protein of the maize (Zea mays L.) Suc synthase1 (sus1) gene were isolated and sequenced. Termini (5' and 3') of the transcribed unit were identified. The SUS1 protein was purified from tissue culture cells as a phosphorylated protein. The overall structure of sus1 is virtually identical with that of the paralogous gene, shrunken1 (sh1); however, the last intron of sh1 is missing in sus1. This intron bears much sequence similarity with the adjacent exon, suggesting that the intron arose from an internal duplication. Although the placement of the other 14 introns is identical in both genes, the introns exhibit markedly greater differences in size and sequence relative to that shown by the exons. An explanation for the differential rate of divergence of exons and introns is selection pressure for gene function. Additionally, comparisons of coding regions of plant sucrose synthases show that sh1-like and sus1-like genes can be found in all monocots so far analyzed. These latter observations point to an important role played by both genes in this group of plants.

Coordinated Transcriptional Regulation of Storage Product Genes in the Maize Endosperm

We have demonstrated that expression of genes involved in starch and storage protein synthesis of the maize (Zea mays L.) endosperm are coordinated. Genetic lesions altering synthetic events in one biosynthetic pathway affect expression of genes in both pathways. Initial studies focused on shrunken2 (sh2) and brittle2 (bt2) mutants because these genes encode subunits of the same enzyme, ADP-glucose pyrophosphorylase. Analysis of various sh2- and bt2- mutant alleles showed that the most severe mutations also conditioned the largest increase in transcripts. The analysis was extended by monitoring the transcripts of the genes, shrunken1 (sh1, structural gene for Suc synthase), sh2, bt2, waxy1 (wx1, structural gene for starch synthase), and those of the large and small zeins in isogenic maize lines at 14, 22, and 30 d postpollination. Endosperms were wild type for all of these genes or contained sh1-, sh2-, bt1-, bt2-, opaque2 (o2-), or amylose-extender1 (ae1-) dull1 (du1-) wx1- mutations. Transcripts increased continually throughout kernel development in the mutants relative to the standard W64A used. Variation in the amount of Suc entering the developing seed also altered transcript amounts. The results indicate that starch and protein biosynthetic genes act in a concerted manner, and both are sensitive to mutationally induced differences.

ADP-glucose pyrophosphorylase in shrunken-2 and brittle-2 mutants of maize

The Shrunken-2 (Sh2) and Brittle-2 (Bt2) genes of maize encode subunits of the tetrameric maize endosperm ADPglucose pyrophosphorylase. However, in all sh2 and bt2 mutants so far examined, measurable ADPglucose pyrophosphorylase activity remains. We have investigated the origin of the residual activity found in various sh2 and bt2 mutants as well as tissue specific expression and post-translational modification of the Sh2 and Bt2 proteins. Sh2 and Bt2 cDNAs were expressed in Escherichia coli and antibodies were prepared against the resulting proteins SH2 and BT2 specific antibodies were used to demonstrate that SH2 and BT2 are endosperm specific, are altered or missing in various sh2 or bt2 mutants, and have a mol. wt. of 54 and 51 kDa respectively in the wild type. The Sh2 and Bt2 transcripts are also endosperm specific. Ten sh2 and eight bt2 mutants show varying severity of phenotypes expressed at transcript, protein subunit and kernel level. Synthesis of multiple transcripts and proteins commonly occurs as a result of sh2 or bt2 mutation. While all mutants produce detectable enzymic activity, not all produce detectable transcripts and proteins. To examine the origin of the apparent non-SH2/BT2 endosperm enzymic activity, homologs of Sh2 and Bt2, designated Agp1 and Agp2 respectively, were isolated from an embryo cDNA library and found to hybridize to endosperm transcripts distinct from those of Sh2 and Bt2. Thus Agp1 and Agp2 or closely related genes may be responsible for the residual activity in some sh2 and bt2 mutants. Surprisingly, no evidence of post-translational modification of the SH2 and BT2 protein subunits was detected.

Expression of ADP-glucose pyrophosphorylase in maize (Zea mays L.) grain and source leaf during grain filling

The time course of ADP-glucose pyrophosphorylase activity and of starch accumulation rate measured in grain, from pollination to maturity, in Zea mays L. plants grown outdoors, was coincident for 2 years. No such correlation was observed in the adjacent leaf, which, furthermore, presented large year-to-year differences in starch accumulation pattern. Analysis of the expression of ADP-glucose synthase at the protein levels, using antibodies directed against the Bt2 or Sh2 subunits, established that the variation of activity in the grain was explained by parallel changes in the content of both subunits. The cDNA for Bt2 and Sh2 subunits were used as probes to quantify the corresponding messenger. In grain, the time course of Bt2 and Sh2 mRNA accumulation anticipated, with a similar pattern, the specific peptide variations, which suggests a transcriptional control of expression. By contrast, the control of leaf activity by protein content was less obvious than in the grain, and changes in leaf enzyme specific activity were suggested during the first 20 d after pollination. A clone homologous to the grain Bt2 subunit cDNA was isolated from a maize leaf cDNA library, and a sequence comparison showed that the leaf clone (L2) was a partial cDNA representing one-third of the mature peptide. A 97% homology was observed between Bt2 and L2 in their coding region, but homology was poor in the 3' noncoding border. This result demonstrates that Bt2 and L2 arise from different genes presenting a tissue-specific expression pattern and provides an explanation for the earlier reported differences between leaf and grain in the size of peptide and mRNA for the Bt2-homologous subunit.

Origin of the main class of repetitive DNA within selected Pennisetum species

In an attempt to identify relationships among genomes of the allotetraploid Pennisetum purpureum Schumach and closely related Pennisetum species with which it can be successfully hybridized, repetitive DNA sequences were examined. Digestion with KpnI revealed two highly repetitive fragments of 140 bp and 160 bp. The possibility that these sequences could be used as genome markers was investigated. Average sequences were determined for the 140 bp and 160 bp KpnI families from P. purpureum and P. squamulatum Fresen. Average sequences (based upon four or five repeats) were determined for the P. glaucum (L.) R. Br. 140 bp KpnI family and the diploid P. hohenackeri Hochst. ex Steud. 160 bp KpnI family. The average sequences of the 160 bp KpnI families in P. purpureum and P. squamulatum differ by only nine bases. The 140 bp KpnI families of the three related species, P. purpureum, P. squamulantum, and P. glaucum are nearly identical, and thus likely represent a recent divergence from a common progenitor or a common genome. Each repetitive sequence may contain internal duplications, which probably diverged following amplification of the original sequence. The 140 bp KpnI repeat probably evolved from the 160 bp KpnI repeat since the missing 18 bp segment is part of the internal duplication that is otherwise conserved in the subrepeats. Tandemly arrayed repetitive sequences in plants are likely to be composed of subrepeats which have been duplicated and amplified.

The Viviparous-1 gene and abscisic acid activate the C1 regulatory gene for anthocyanin biosynthesis during seed maturation in maize

The Viviparous-1 (Vp1) gene is required for expression of the C1 regulatory gene of the anthocyanin pathway in the developing maize seed. We show that VP1 overexpression and the hormone, abscisic acid (ABA), activate a reporter gene driven by the C1 promoter in maize protoplasts. Cis-acting sequences essential for these responses were localized. Mutation of a conserved sequence in the C1 promoter abolishes both ABA regulation and VP1 trans-activation. An adjacent 5-bp deletion blocks ABA regulation but not VP1 trans-activation. The latter mutant reconstructs the promoter of c1-p, an allele that is expressed during seed germination but not during seed maturation. We suggest that VP1 activates C1 specifically during maturation by interacting with one or more ABA-regulated transcription factors.

One of two different ADP-glucose pyrophosphorylase genes from potato responds strongly to elevated levels of sucrose

The key regulatory step in starch biosynthesis is catalyzed by the tetrameric enzyme ADP-glucose pyrophosphorylase (AGPase). In leaf and storage tissue, the enzyme catalyzes the synthesis of ADP-glucose from glucose-1-phosphate and ATP. Using heterologous probes from maize, two sets (B and S) of cDNA clones encoding potato AGPase were isolated from a tuberspecific cDNA library. Sequence analysis revealed homology to other plant and bacterial sequences. Transcript sizes are 1.9 kb (AGPase B) and 2.1 kb (AGPase S). Northern blot experiments show that the two genes differ in their expression patterns in different organs. Furthermore, one of the genes (AGPase S) is strongly inducible by metabolizable carbohydrates (e.g. sucrose) at the RNA level. The accumulation of AGPase S mRNA was always found to be accompanied by an increase in starch content. This suggests a link between AGPase S expression and the status of a tissue as either a sink for or a source of carbohydrates. By contrast, expression of AGPase B is much less variable under various experimental conditions.

Identification and molecular characterization of shrunken-2 cDNA clones of maize

Mutation at the shrunken-2 (Sh2) locus of maize, a gene described more than 40 years ago, greatly reduces starch levels in the endosperm through its effect on the starch synthetic enzyme ADP-glucose pyrophosphorylase, an enzyme thought to be regulatory in this biosynthetic pathway. Although our previous work has suggested that Sh2 is a structural gene for this enzyme, we have also reported data compatible with Sh2 acting post-transcriptionally. In this study, we took advantage of a transposable element-induced Sh2 allele, its progenitor, and revertants to identify a clone for this locus. Although the cloning and identification were done independently of any knowledge concerning the product of this gene, examination of the deduced amino acid sequence revealed much similarity to known ADP-glucose pyrophosphorylase subunits of plants and bacteria, including regions involved in substrate binding and activator binding. Little sequence similarity, however, was found at the DNA level. These observations provide direct evidence that Sh2 encodes a subunit for endosperm ADP-glucose pyrophosphorylase. Analysis of several phenotypically wild-type alleles arising from a mutable sh2-Ds allele revealed one unexpected case in which DNA sequences of Sh2 were rearranged in comparison with the progenitor Sh2. In contrast to wild type, the Ds-induced sh2 allele conditions at least two transcripts in the endosperm.

Increased gene expression by the first intron of maize shrunken-1 locus in grass species

The first intron of the shrunken-1 (Sh1) locus of maize was incorporated into constructs containing the chloramphenicol acetyltransferase gene (CAT) coupled with the nopaline synthase 3' polyadenylation signal. Transcription was driven with the 35S promoter of the cauliflower mosaic virus (CaMV) or the Sh1 promoter of maize. Transient gene expression was monitored following electroporation into protoplasts of Panicum maximum (guineagrass), Pennisetum purpureum (napiergrass), or Zea mays (maize). The 1028 base pair intron increased gene expression in cells of each species when transcription was driven with the 35S promoter. Eleven to 91-fold increases were observed. Expression levels observed in maize were two and eight times those observed in napiergrass and guineagrass, respectively. The 35S promoter gave CAT activity 10 to 100 times that observed with the Sh1 promoter. Whereas expression driven by the 35S promoter was reproducible, that observed with the Sh1 promoter proved quite variable. In similar constructs the first intron of the alcohol dehydrogenase-1 (Adh1) gene of maize led to increased gene expression of only 7 to 10% of that observed with the Sh1 first intron. The increased level of gene expression caused by the Sh1 first intron is approximately 10 times higher than that caused by any other plant introns that have been used. Thus, the Sh1 first intron may prove quite useful in increasing expression of foreign genes in monocots and possibly other plants.

Promoter strength comparisons of maize shrunken 1 and alcohol dehydrogenase 1 and 2 promoters in mono- and dicotyledonous species

Promoter strengths of two maize alcohol dehydrogenase genes, Adh1 and Adh2, and the maize shrunken-1 gene, Sh1, were evaluated by transient expression in cultured protoplasts of Panicum maximum, Triticum monococcum, and Daucus carota. Promoter elements were ligated in correct and opposite orientations as transcriptional gene fusions to the chloramphenicol acetyl transferase gene containing the nopaline synthase 3' polyadenylation signal. The relative levels of gene expression were compared to the cauliflower mosaic virus 35S promoter. The full length Adh1 promoter (-1100 to +15) functioned in all species, but at a reduced level in D. carota. An Adh1 promoter deletion from -304 to -1100 did not express at detectable levels in any species nor did the Sh1 promoter construction. The Adh2 promoter (-860 to +90) only expressed in D. carota. The full length Adh1 promoter gave the highest level of CAT expression in the monocot cells but at levels which were approximately 30% compared to the CaMV 35S promoter. This was reduced further in D. carota to approximately 4%. These data suggest that at least some of the regulatory factors responsible for promoter function are somewhat species specific and that these differences should be considered in gene expression studies.

Shrunken-1 encoded sucrose synthase is not required for sucrose synthesis in the maize endosperm

Kernels of wild-type maize (Zea mays L.) shrunken-1 (sh1), deficient in the predominant form of endosperm sucrose synthase and shrunken-2 (sh2), deficient in 95% of the endosperm ADP-glucose pyrophosphorylase were grown in culture on sucrose, glucose, or fructose as the carbon source. Analysis of the endosperm extracts by gas-liquid chromatography revealed that sucrose was present in the endosperms of all genotypes, regardless of carbon supply, indicating that all three genotypes are capable of synthesizing sucrose from reducing sugars. The finding that sucrose was present in sh1 kernels grown on reducing sugars is evidence that shrunken-1 encoded sucrose synthase is not necessary for sucrose synthesis. Shrunken-1 kernels developed to maturity and produced viable seeds on all carbon sources, but unlike wild-type and sh2 kernels grown in vitro, sucrose was not the superior carbon source. This latter result provides further evidence that the role of sucrose synthase in maize endosperm is primarily that of sucrose degradation.

The mutation bronze-mutable 4 derivative 6856 in maize is caused by the insertion of a novel 6.7-kilobase pair transposon in the untranslated leader region of the bronze-1 gene

The Ds-controlled allele, bz-m4 Derivative 6856 [bz-m4 D6856], is reported to have an altered temporal- and tissue-specific pattern of gene expression. We have cloned this allele and have characterized it at the molecular level. The mutation was caused by the insertion of a complex transposon-like structure 36 base pairs downstream from the Bz mRNA cap site. The insert is 6.7-kbp long. Ds elements, each approximately 2 kbp in length, are at both ends of the insert. The sequence between the Ds elements is a partial duplication of flanking sequences from the 3' end of the Bz gene. These data suggest that Ds initially inserted near the 3' end of the gene and mobilized adjacent sequences as it transposed.

Sugar utilization by developing wild type and shrunken-2 maize kernels

To characterize the movement of sugars during kernel development in maize, a newly devised in vitro kernel development scheme was utilized. Viable seeds of wild type maize (Zea mays L.) as well as the mutant shrunken-2 (sh2) were found to mature when grown in culture with reducing sugars or sucrose as the carbon source. However, wild type and sh2 kernels had greater germination, starch content, and seed weight when sucrose, rather than reducing sugars, was the carbon source. By the use of labeled sucrose it was shown that sucrose can move into endosperm tissue without intervening degradation and resynthesis. These results show that when grown in vitro the maize seed can utilize reducing sugars for development, but it prefers sucrose.

Development of wild type, shrunken-1 and shrunken-2 maize kernels grown in vitro

Kernels of wild type maize (Zea mays L.) and the mutants shrunken-1 and shrunken-2 developed as much as in vivo when excised at five days post-pollination and grown in culture using existing methods. Mature kernels from culture exhibited their expected phenotypes. Starch, sugar and enzyme levels of kernels grown in culture were similar to those known to occur in kernels of the same genotypes grown in vivo. Differences in percentage germination of kernels grown in vitro were similar to those of kernels grown in vivo.

ALTERED MAIZE ENDOSPERM ADP-GLUCOSE PYROPHOSPHORYLASES FROM REVERTANTS OF A SHRUNKEN-2-DISSOCIATION ALLELE

Four phenotypically wild-type seeds were obtained from separate Activator-induced events in the Dissociation-inhibited allele sh2-ml (shrunken-2, mutable-1). Endosperm adenosine diphosphoglucose pyrophosphorylase, the enzyme controlled by sh2, was extracted and partially purified from the four revertants and was compared to enzyme produced by the progenitor Sh2 allele and the sh2-m allele.

The revertants contained 50 to 140% of the activity conditioned by the progenitor allele. Each of the revertants appears to be unique as judged by differences in Km(glucose-1-PO4), 3-phosphoglycerate(3-PGA) activation, and phosphate-inhibition. In one case the reversion event apparently increased the sensitivity of ADP-glucose pyrophosphorylate to 3-PGA activation.

Comparison of chemical and microbiological methods in the estimation of methionine in cowpea (Vigna unguiculata) seeds

1. Meals were prepared from the seeds of fifteen varieties of cowpea (Vigna unguiculata), one of lima bean (Phaseolus lunatus) and one of yam bean (Sphenostylis stenocarpa), and their methionine content was determined by six different methods. 2. Total methionine content was determined by two chemical methods (ion-exchange chromatography and a colorimetric procedure) and by two microbiological methods. The 'available' methionine content was determined by microbiological assay with Streptococcus zymogenes. 3. All the different methods for total methionine determination gave similar results, with much the same high extent of precision. 4. The value for 'available' methionine content were similar to or marginally higher than the corresponding microbiological assay value for total methionine content. There was no indication that the methionine in any of the test samples was not completely available.

Characterization of ADP-glucose pyrophosphorylase from shrunken-2 and brittle-2 mutants of maize

Electrophoretic characterization of adenosine diphosphate glucose pyrophosphorylase from the developing endosperms of nine shrunken-2 and four brittle-2 mutants revealed that (1) all mutants had low but detectable levels of activity, (2) mutation at either locus decreased activity of pyrophosphorylases A and B, and (3) differences in mobility were not found. However, pyrophosphorylase B extracted from several shrunken-2 and brittle-2 mutants differed from normal in extent of urea denaturation, Km (glucose-1-phosphate) or type of glucose-1-phosphaociation with the sh2 locus) appears to differ from normal in Km (glucose-1-phosphate).

Characterization of adenosine diphosphate glucose pyrophosphorylases from developing maize seeds

Electrophoretic examination of 22-day-old, normal maize (Zea mays L.) endosperm extracts revealed two zones of adenosine diphosphate glucose pyrophosphorylase activity. The enzymes are identical in terms of Km for glucose 1-phosphate and the effect of 3-phosphoglyceric acid on apparent Km for glucose 1-phosphate. Both enzymatic activities increase with increasing doses of the functional alleles at the shrunken-2 and brittle-2 loci. Molecular weight differences between the two electrophoretic species were inferred from sucrose gradient centrifugation. It is suggested that the two bands of activity represent different aggregation states of the same enzyme because under different extraction conditions, only one enzyme is found. Molecular weight estimates of 237,000 and 253,000 were obtained for the smaller enzyme. It is suggested that this enzyme is an aggregate of several subunits. Comparison of the embryo and endosperm pyrophosphorylases showed the embryo activity to be more heat stable and probably independent of direct shrunken-2 or brittle-2 control.