Inactivation of maize transposon Mu suppresses a mutant phenotype by activating an outward-reading promoter near the end of Mu1

Mutator insertions in an intron of the maize knotted1 gene result in dominant suppressible mutations

The knotted1 (kn1) locus of maize is defined by a series of dominant mutations affecting leaf development. We recovered 10 additional mutant alleles in lines containing active Mutator transposable elements. Nine of these alleles contain Mu1 or Mu8 elements inserted within a 310-bp region of the kn1 third intron. All five Mu8 insertions are in the same orientation whereas both orientations of Mu1 were recovered. Northern analysis showed that ectopic expression of kn1 within developing leaves is correlated with the mutant phenotype for the four alleles analyzed. Transcript size was not altered. The effect of Mu activity, as measured by the extent of Mu element methylation or by the presence of the autonomous MuDR element, was investigated for two alleles. Kn1-mum2, containing a Mu8 element, and Kn1-mum7, containing a Mu1 element, required Mu activity for the knotted phenotype. We examined the effect of Mu activity on ectopic kn1 expression in Kn1-mum2 and found that the transcript was present in leaves of Mu active individuals only. We discuss possible mechanisms by which Mu activity could condition kn1 gene expression.

Large tandem duplication associated with a Mu2 insertion in Zea mays B-Peru gene

The b locus of Zea mays encodes a transcriptional activator of the anthocyanin biosynthetic pathway. The B-Peru allele is expressed in the aleurone layer of the seed, which results in dark purple pigmentation of this tissue. An unstable Mutator-induced B-Peru mutant allele, b-Perum220, displays weak, variable pigment and a high germinal reversion rate not characteristic of other Mutator insertions. Characterization of relevant regions of b-Perum220 revealed a Mu2 element insertion in one copy of a 534 bp sequence. This 534 bp sequence is tandemly triplicated in the progenitor B-Peru allele, upstream of the B-Peru transcription start site. In addition to the Mu2 insertion, the b-Perum220 allele contains a newly formed large tandem duplication of 4.0 kb, which includes the promoter region and the first three exons of the B-Peru gene. The Mu2 element does not reside at any of the duplication breakpoints. The molecular study of eleven independent germinal revertants revealed five structural classes including structures in which the 4.0 kb tandem duplication is partially or completely deleted, the Mu2 element is partially or completely deleted, or a combination of these events has occurred. We hypothesize that most of the revertants arose by unequal recombination between the duplicated regions. Based on these structural analyses, models are discussed to explain the reduced b gene expression in b-Perum220.

Ds read-out transcription in transgenic tomato plants

To select for Ds transposition in transgenic tomato plants a phenotypic excision assay, based on restoration of hygromycin phosphotransferase (HPT II) gene expression, was employed. Some tomato plants, however, expressed the marker gene even though the Ds had not excised. Read-out transcriptional activity of the Ds element is responsible for the expression of the HPT II gene. Transcription initiation was mapped to multiple positions spanning about 300 bp in the subterminal part of the Ds element. In this respect Ds in tomato resembles the maize element Mu1, which also promotes transcription outward from the element. Transposon read-out transcription might thus supply an additional general mechanism for controlling plant gene expression.

Coordinate suppression of mutations caused by Robertson's mutator transposons in maize

Transposable elements from the Robertson's Mutator family are highly active insertional mutagens in maize. However, mutations caused by the insertion of responder (non-autonomous) elements frequently depend on the presence of active regulator (autonomous) elements for their phenotypic effects. The hcf106::Mu1 mutation has been previously shown to depend on Mu activity in this way. The dominant Lesion-mimic 28 mutation also requires Mu activity for its phenotypic effects. We have used double mutants to show that the loss of Mu activity results in the coordinate suppression of both mutant phenotypes. This loss can occur somatically resulting in large clones of cells that have a wild-type phenotype. Autonomous and non-autonomous Mutator elements within these clones are insensitive to digestion with methylation-sensitive enzymes, suggesting extensive methylation of CG and non-CG cytosine residues. Our data are consistent with the sectors being caused by the cycling of MuDR regulatory elements between active and inactive phases. The pattern of sectors suggests that they are clonal and that they are derived from the apical cells of the vegetative shoot meristem. We propose that these cells are more likely to undergo epigenetic loss of Mu activity because of their longer cell division cycle during shoot growth. Coordinate suppression of unlinked mutations can be used to perform mosaic analysis in maize.

Genetic analysis of 63 mutations affecting maize kernel development isolated from Mutator stocks

Sixty-three mutations affecting development of the maize kernel were isolated from active Robertson's Mutator (Mu) stocks. At least 14 previously undescribed maize gene loci were defined by mutations in this collection. Genetic mapping located 53 of these defective kernel (dek) mutations to particular chromosome arms, and more precise map determinations were made for 21 of the mutations. Genetic analyses identified 20 instances of allelism between one of the novel mutations and a previously described dek mutation, or between new dek mutations identified in this study; phenotypic variability was observed in three of the allelic series. Viability testing of homozygous mutant kernels identified numerous dek mutations with various pleiotropic effects on seedling and plant development. The mutations described here presumably arose by insertion of a Mu transposon within a dek gene; thus, many of the affected loci are expected to be accessible to molecular cloning via transposon-tagging.

Expression of the two maize TATA binding protein genes and function of the encoded TBP proteins by complementation in yeast

A single gene encodes the TATA binding protein (TBP) in yeasts and animals. Although two TBP-encoding genes (Tbp) previously were isolated from both Arabidopsis and maize, the expression and in vivo function of the encoded plant TBPs were not investigated. Here, we report that the two highly conserved maize Tbp genes are unlinked and reside within larger, ancestrally duplicated segments in the genome. We find quantitative differences in Tbp1 versus Tbp2 transcript accumulation in some maize tissues. These nonidentical expression patterns may indicate differences in the tissue-specific regulation of these genes, which might allow the two encoded maize TBP isoforms to perform nonoverlapping functions in the plant. In addition, we show that the maize TBP products, unlike animal TBPs, are functionally interchangeable with yeast TBP for conferring yeast cell viability. This is a conclusive demonstration of in vivo activity for a nonyeast TBP protein, and these complementation results point to particular amino acids in TBP that are likely to influence species-specific protein interactions.

Reductase activity encoded by the HM1 disease resistance gene in maize

The HM1 gene in maize controls both race-specific resistance to the fungus Cochliobolus carbonum race 1 and expression of the NADPH (reduced form of nicotinamide adenine dinucleotide phosphate)-dependent HC toxin reductase (HCTR), which inactivates HC toxin, a cyclic tetrapeptide produced by the fungus to permit infection. Several HM1 alleles were generated and cloned by transposon-induced mutagenesis. The sequence of wild-type HM1 shares homology with dihydroflavonol-4-reductase genes from maize, petunia, and snap-dragon. Sequence homology is greatest in the beta alpha beta-dinucleotide binding fold that is conserved among NADPH- and NADH (reduced form of nicotinamide adenine dinucleotide)-dependent reductases and dehydrogenases. This indicates that HM1 encodes HCTR.

Active Mutator elements suppress the knotted phenotype and increase recombination at the Kn1-O tandem duplication

The KNOTTED-1 (KN1) locus is defined by a number of dominant mutations that affect leaf development. The Kn1-O mutation is characterized by outpocketings of tissue along lateral veins of the maize leaf and by displacement of ligule tissue from the junction of the blade and sheath into the blade. Kn1-O results from a tandem duplication of 17 kb; each repeat includes the entire 8-kb KN1 transcription unit. Mutator (Mu) transposable elements inserted at the junction of the two repeats diminish the mutant phenotype. The Mu insertions affect the Kn1-O mutation in several distinctive ways. (1) Two of the three Mu elements, a Mu1 and a Mu8 element, diminish the mutant phenotype only when active as indicated by hypomethylation; when methylated or inactive, the phenotype is comparable to the Kn1-O progenitor. (2) Additional rearrangements have arisen in these derivatives that further reduce the mutant phenotype. (3) A 100-2000-fold increase in the loss of one repeat occurs in the presence of Mu elements as compared to Kn1-O without elements. The high frequency of loss only occurs when the Mu elements are hypomethylated. The frequency is also influenced by the specific allele carried at the same locus on the homologous chromosome. Reciprocal exchange of flanking markers does not accompany the loss events. Various recombination models that address the events occurring at Kn1-O are presented.