An insertion sequence of 283 base pairs has been isolated from the DFR-C gene (dihydroflavonol-4-reductase) of petunia. This insert was found only in a line unstable for the An1 locus (anthocyanin 1, located on chromosome VI) and not in fully pigmented progenitor and revertant lines or in stable white derivative lines. This implies that the An1 locus encodes the DFR-C gene. The unstable An1 system in the line W138 is known to be a two-element system, the autonomous element being located on chromosome I. In the presence of the autonomous element, W138 flowers exhibit a characteristic pattern of red revertant spots and sectors on a white background. In the absence of the autonomous element, the W138 allele gives rise to a stable recessive (white) phenotype. Sequence analysis of progenitor, unstable, and revertant alleles revealed dTph1 to contain perfect terminal inverted repeats of 12 base pairs. In DFR-C, it is flanked by an 8-base pair target site duplication. Sequences homologous to dTph1 are present in at least 50 copies in the line W138. Sequence analysis of An1 revertant alleles indicated that excision, including removal of the target site duplication, is required for reversion to the wild-type phenotype. Derivative stable recessive alleles showed excision of dTph1 and a rearrangement of the target site duplication. dTph1 is the smallest transposable element described to date that is still capable of transposition. The use of dTph1 in tagging experiments and subsequent gene isolation is discussed.
SUMMARYThe expression of two types of sucrose synthase-encoding genes, Ssl and Ss2, in hexaploid wheat (Triticum aestivum, L.), has been investigated using type-specific probes, corresponding to the 250-270 bp C-terminal portions of the respective cDNA clones. Both types of genes are highly expressed in developing endosperm, where the expression of the Ss2 type slightly precedes in time that of the Ssl type. Expression of Ss genes is lower in etiolated leaves and in roots than in endosperm. In the first two tissues, the Ssl mRNA is much more abundant than the Ss2 mRNA, and the Ssl mRNA level sharply increases in response to anaerobiosis and to cold shock (6°C), while the level of Ss2 mRNA is not significantly affected. Upon illumination of etiolated leaves, the Ssl level mRNA decreases significantly and the Ss2 mRNA level increases.
We have characterized three cDNA clones corresponding to proteins CM1, CM3 and CM16, which represent the three types of subunits of the wheat tetrameric inhibitor of insect a-amylases. The deduced amino acid sequences of the mature polypeptides are homologous to those of the dimeric and monomeric a-amylase inhibitors and of the trypsin inhibitors. The mature polypeptides are preceded by typical signal peptides. Southern blot analysis of appropriate aneuploids, using the cloned cDNAs as probes, has revealed the location of genes for subunits of the CM3 and of the CM16 type within a few kb of each other in chromosomes 4A, 4B and 4D, and those for the CM1 type of subunit in chromosomes 7A, 7B and 7D. Known subunits of the tetrameric inhibitor corresponding to genes from the B and D genomes have been previously characterized. No proteins of this class have been found to be encoded by the A genome in hexaploid wheat (genomes AA, BB, DD) or in diploid wheats (AA) and no anti a-amylase activity has been detected in the latter, so that the A-genome genes must be either silent (pseudogenes) or expressed at a much lower level.
A new type of neutral thionin (type V), specifically expressed in developing wheat endosperm, has been found to be encoded by a set of single-copy genes located in the long arms of chromosomes 1A, IB and ID, within less than 10,000 base-pairs of those corresponding to the highly basic type-I thionins. Divergence between types I and V has occurred through a process of accelerated evolution that has affected the amino acid sequence of the mature thionin but not the precursor domains corresponding to the N-terminal signal peptide and the long C-terminal acidic peptide. This process in volved a deletion and a non-synonymous nucleotide substitution rate equal to the synonymous rate in the thionin sequence.
SUMMARYA cDNA library from developing wheat endosperm was screened for sucrose-synthase clones using a maize cDNA probé corresponding to the Shl locus under non-stringent conditions. Five positive clones were isolated and initially classified into two types on the basis of their relative ability to hybridize with the probé and of their partial restriction maps. Determination of the nucleotide sequences indicated homology between the two types of wheat clones, with type 1 showing higher homology to the maize Shl locus than to type-2 sequences. The inserís cloned in plasmids pST8 (type 1) and pST3 (type 2) were used as probes to determine the chromosomal locations of the two types of genes. DNAs from compensated nulli-tetrasomic and ditelosomic lines of wheat cultivar Chinese Spring were cleaved with EcoRl and analysed in Southern blots. DNA segments of the two types were thus identified in the short arms of chromosomes 7A, 7D, and, possibly, 7B. The two types of linked loci have been designated Ssl and Ss2, respectively.
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