A novel wheat α-amylase gene (α-Amy3)

A genomic clone of a wheat alpha-amylase gene (lambdaAmy3/33) was identified, on the basis of hybridisation properties, as different from alpha-Amy1 and alpha-Amy2 genes which had been characterised previously. The nucleotide sequence revealed that this gene has the normal sequence motifs of an active gene and an open reading frame interrupted by two introns. The protein sequence encoded by this open reading frame is recognisably similar to that of alpha-amylase from the alpha-Amy1 and alpha-Amy2 genes and there is high sequence homology in all three proteins at the putative active sites and Ca++ binding region. In addition, the introns are at positions equivalent to the position of introns in the alpha-Amy1 and alpha-Amy2 genes. However, the sequence was less similar to alpha-Amy1 and alpha-Amy2 than these are to each other. Southern blot analysis showed that the lambdaAmy3/33 DNA is one of a small multigene family carried on a different chromosome (group 5) from either the alpha-Amy1 or alpha-Amy2 genes. A further difference from the alpha-Amy1 and alpha-Amy2 genese was the pattern of expression. lambdaAmy3/33 was expreseed only in immature grains and, unlike the alpha-Amy1 and alpha-Amy2 genes, not at all in germinating aleurones. These data suggested therefore that this gene represents a third type of alpha-amylase gene, not described before, which shares a common evolutionary ancestor with the alpha-Amy1 and alpha-Amy2 genes.

Synthesis and secretion of wheat alpha-amylase in Saccharomyces cerevisiae

A wheat alpha-amylase cDNA clone has been fused to the phosphoglycerate kinase initiator methionine to enable synthesis in the yeast Saccharomyces cerevisiae of an alpha-amylase enzyme that is identical in size to the wild-type alpha-amylase. The alpha-amylase is synthesized with an N-terminal plant signal peptide which is recognized in the yeast host, leading to efficient processing and secretion into the medium. The secretion of alpha-amylase into the medium is quite efficient in rich medium, but barely detectable in a minimal medium.

Expression of α-amylase and other gibberellin-regulated genes in aleurone tissue of developing wheat grains

Aleurone tissue from freshly harvested immature wheat grains (Triticum aestivum L. cv. Sappo) which is normally unresponsive to gibberellic acid can be made responsive by subjecting the tissue to a pre-incubation treatment in a simple buffered medium prior to the addition of the growth substance. The effectiveness of this treatment is dependent on grain age, with grains less than 15-20 days post anthesis failing to become converted to a responsive state whilst tissue from grains older than this become increasingly susceptible. Tissue from grains of a certain age (approx. 25-28 days post anthesis) produce small amounts of α-amylase following this treatment even in the absence of exogenously applied growth substance. Using different (32)-labelled complementary-DNA probes for α-amylase in wheat it was demonstrated that the failure of freshly harvested tissue to produce α-amylase was correlated with the absence of the appropriate mRNA species. Inability to accumulate α-amylase mRNA in response to gibberellic acid was removed by the pre-iccubation treatment and also by enforced drying. The gibberellin-regulated expression of other unidentified genes also responds to pre-incubation or drying. Induction of gibberellin-responsiveness in immature aleurone cells did not extend to the secretion of acid phosphatase, protease and ribonuclease.

The organisation and interviral homologies of genes at the 3′ end of tobacco rattle virus RNA1

The RNA1 of tobacco rattle virus (TRV) has been cloned as cDNA and the nucleotide sequence determined of 2 kb from the 3'-terminal region. The sequence contains three long open reading frames. One of these starts 5' of the cDNA and probably corresponds to the carboxy-terminal sequence of a 170-K protein encoded on RNA1. The deduced protein sequence from this reading frame shows homology with the putative replicases of tobacco mosaic virus (TMV) and tricornaviruses. The location of the second open reading frame, which encodes a 29-K polypeptide, was shown by Northern blot analysis to coincide with a 1.6-kb subgenomic RNA. The validity of this reading frame was confirmed by showing that the cDNA extending over this region could be transcribed and translated in vitro to produce a polypeptide of the predicted size which co-migrates in electrophoresis with a translation product of authentic viral RNA. The sequence of this 29-K polypeptide showed homology with two regions in the 30-K protein of TMV. This homology includes positions in the TMV 30-K protein where mutations have been identified which affect the transport of virus between cells. The third open reading frame encodes a potential 16-K protein and was shown by Northern blot hybridisation to be contained within the region of a 0.7-kb subgenomic RNA which is found in cellular RNA of infected cells but not virus particles. The many similarities between TRV and TMV in viral morphology, gene organisation and sequence suggest that these two viral groups may share a common viral ancestor.

Expression of a wheat alpha-amylase gene in Escherichia coli: recognition of the translational initiation site and the signal peptide

Transcription of a full-length cDNA clone of wheat alpha-amylase using a lac promoter in Escherichia coli results in synthesis of a precursor alpha-amylase polypeptide of the correct size, indicating that translation initiates correctly. Recognition of the plant translational initiation site by E. coli ribosomes is 15-20% as efficient as the ribosome-binding site of the beta-lactamase gene when it is fused to alpha-amylase. The alpha-amylase signal peptide is recognised in E. coli resulting in secretion of the enzyme into the periplasmic space; deletion of the signal peptide prevents secretion. Replacement of the alpha-amylase signal peptide with a beta-lactamase signal peptide also enables the bacterial cell to secrete the enzyme. The presence of the beta-lactamase and the alpha-amylase signal peptides in tandem results in secretion of the enzyme and removal of both signal peptides.

α-amylase genes of wheat are two multigene families which are differentially expressed

The α-Amy1 and α-Amy2 genes of wheat produce distinct subsets of α-amylase isozymes which show different patterns of expression in wheat aleurone cells and in developing grain. In order to characterise the organisation and expression of these genes, clones of α-Amy1 and α-Amy2 cDNA have been isolated. The two types of cDNA clone were distinguished within a small library of α-amylase cDNA clones (Baulcombe and Buffard, Planta 157 493-501 [1983]) by restriction endonuclease mapping and by cross hybridisation. The identity of α-Amy1 or α-Amy2 type was assigned from the results of hybrid selected translation analysis in which small subfragments of the cDNA clones were used. These subfragments were derived from the 3' ends of the cDNA and did not cross hybridise between the different types of cDNA. Hybridisation of α-Amy1 and α-Amy2 cDNA probes to restriction enzyme digests of wheat nuclear DNA revealed that these are multigene families located on the group 6 (α-Amy1) and group 7 (α-Amy2) chromosomes. Studies on the levels of α-Amy1 and α-Amy2 mRNA in developing grain and in aleurone tissue indicated that the differences in isozyme expression are due to the patterns of mRNA accumulation. In aleurone tissue the α-Amy1 transcripts accumulate in parallel with other genes which are regulated by gibberellic acid, while the accumulation of α-Amy2 genes is sustained for 36 h longer.

Gibberellic-acid-regulated expression of α-amylase and six other genes in wheat aleurone layers

The effect of gibberellic acid (GA3) on gene expression in wheat aleurone cells has been characterised. In-vitro translation of polyadenylated RNA indicated that α-amylase and other messenger-RNA (mRNA) species increase in relative concentration in GA3-treated tissue. At least one mRNA species declines in relative level in response to GA3. There is also a GA3-dependent, four-fold increase in the level of polyadenylated RNA. This effect is largely the result of increased levels of many mRNA species which are also present in untreated tissue. Seven GA3-induced polyadenylated RNA species including the Amyl α-amylase gene product have been cloned as complementary DNA in the plasmid pBR322. These cloned DNAs have been used as hybridisation probes to show that the GA3-induced increase in α-amylase mRNA is more prolonged than the accumulation of the other GA3-regulated mRNA species. A polyadenylated-RNA sequence showing reduced concentration in GA3-treated tissue has also been cloned.

Polyadenylated RNA sequences which are reduced in concentration following auxin treatment of soybean hypocotyls

Previous work has shown that any effect of exogenous auxin on gene expression in soybean hypocotyl tissue must be restricted to a relatively small fraction of the polyadenylated RNA. However, kinetic hybridization analysis with cDNA probes revealed that a minor abundant class of sequences is markedly reduced in concentrations in the auxin-treated polyadenylated RNA. Recombinant plasmids containing copies of polyadenylated RNA species were constructed using the G-C tailing procedure and clones of auxin-regulated sequences were detected by differential in situ hybridization with cDNA of polyadenylated RNA from auxin-treated or untreated hypocotyls. Although the 12 clones which were selected all contained different size inserts, and were therefore independent, 11 of these apparently hybridized to just two different RNA species. The rate constant of the auxin-sensitive abundant component of the untreated polyadenylated RNA/DNA hybridization was similar to that of the reaction between the two major groups of clones and untreated polyadenylated RNA. This indicates that these cloned sequences are homologous with that cDNA fraction. The twelfth clone is thought to be representative of a group of less abundnt auxin-regulated polyadenylated mRNA species which had been detected in an earlier analysis of the in vitro translation products of soybean hypocotyl RNA. Both the timing and the extent of the influence of auxin on the relative concentration of these cloned sequences are quite consistent with a close relationship between growth regulation by auxin and its effects on gene expression.