A 168 bp derivative of Suppressor-mutator/Enhancer is responsible for the maize o2-23 mutation

Transcription and somatic transposition of the maize En/Spm transposon system in rice

Transposition of the maize En/Spm system in rice was investigated using a two-component construct consisting of an immobilised transposase source driven by the CaMV 35S-promoter, and a modified I/dSpm transposon. Mobilization of I/dSpm in somatic sectors was demonstrated by sequencing of excision products and isolation of flanking genomic sequences in T0 and T1 progeny plants. Since the transposition efficiency appeared to be considerably lower than that observed in maize or in other heterologous systems like Arabidopsis, we examined En/Spm transcription and splicing in the transgenic rice plants. Northern analysis revealed the presence of transcripts encoding the active TnpA and TnpD transposases, with the latter predominating; this is the reverse of what is seen in maize and Arabidopsis. RT-PCR analysis confirmed the occurrence of correct splicing and the formation of the two other alternatively spliced transcripts (TnpB and TnpC), as previously described for maize. Two alternative splice donor sites at the end of exon 1 were identified in maize at positions 578 and 704. We observe that rice is similar to maize in that TnpA is preferentially spliced at position 578. We also show that in Arabidopsis splicing occurs preferentially at position 704, as in other dicots like tobacco. These observations indicate differences in the splicing of transcripts of the maize En/Spm element between dicot and monocot hosts. Nevertheless, the ratio in which the transcripts for the active transposases are produced seems to determine the efficiency of transposition, irrespective of the host considered. A limiting amount of TnpA might therefore be responsible for the lower transposition activity of En/Spm in rice. Alternatively, reduced mobility of the modified I/dSpm element used may have resulted from the absence of critical sequences necessary for transposition. The influence of endogenous, autonomous, En/Spm -related elements present in the rice genome on the transposition behaviour of the exogenous maize element is also considered.

Specific combinations of zein genes and genetic backgrounds influence the transcription of the heavy-chain zein genes in maize opaque-2 endosperms

The transcript levels of heavy-chain zein genes (zH1 and zH2) and the occurrence of the zH polypeptides in different opaque-2 (o2) lines were investigated by RNA-blot analyses and by sodium dodecylsulfate-polyacrylamide gel electrophoresis or two-dimensional gel electrophoresis protein fractionations. Four mutant alleles o2R, o2T, o2It, and o2-676 introgressed into different genetic backgrounds (GBs) were considered. The mono-dimensional gel electrophoresis zein pattern can be either conserved or different among the various GBs carrying the same o2 allele. Likewise, in the identical GB carrying different o2 alleles, the zein pattern can be either conserved or differentially affected by the different mutant allele. Zein protein analysis of reciprocal crosses between lines with different o2 alleles or the same o2 showed in some case a more than additive zH pattern in respect to the o2 parent lines. Electrophoretic mobility shift assay approaches, with O2-binding oligonucleotide and endosperm extracts from the above o2 lines, failed to reveal o2-specific retarded band in any of the o2 extracts. The results suggest that the promoter of some zH1 and zH2 contains motif(s) that can respond to factors other than O2.

The activity of the maize Opaque2 transcriptional activator is regulated diurnally

The maize (Zea mays L.) Opaque2 (O2) protein is an endosperm-specific transcriptional activator whose DNA-binding activity is regulated diurnally by a phosphorylation/dephosphorylation mechanism. We show that the O2 transcript undergoes pronounced oscillations during the day-night cycle. The highest level of the O2 message is present at midday and the lowest level at midnight. The level of O2 transcript follows a diurnal rhythm that appears controlled by the circadian clock. Two different endosperm-expressed DNA-binding proteins, PBF (prolamin box-binding factor) and OHP1 (O2-heterodimerizing protein 1), were also analyzed. While the PBF message levels oscillate diurnally, the steady-state levels of OHP1 transcript were constant through the day and night. We present data showing that the seed is not directly involved in the perception of the light signal, but presumably responds to diurnal fluxes of nutrients into the endosperm. Moreover, we show that the O2 protein is not involved in the regulation of its own transcript levels. These data indicate that O2 activity is down-regulated at night by both a reduction in O2 transcript and by hyperphosphorylation of residual O2 protein, and suggest that regulatory gene activity during endosperm development may be acutely sensitive to a diurnal signal(s) emanating from the plant and passing into the developing seeds.

Analysis of the methylation pattern of the maize opaque-2 (O2) promoter and in vitro binding studies indicate that the O2 B-Zip protein and other endosperm factors can bind to methylated target sequences

The maize opaque-2 locus (o2) has an endosperm-specific expression and is positively autoregulated by its gene product, a b-Zip protein, to a TGACGTTG motif. The genomic sequencing method was used here to describe, in leaf and endosperm, the methylation pattern of a 390-base pair region of the o2 promoter. In leaf, 96% of the C residues are methylated, whereas in endosperm the 5-methylcytosine content is 84%. Comparison of these methylation patterns indicates that the o2 tissue-specific expression does not result from the demethylation of any specific C residue and that, in vivo, O2 interacts with a methylated target sequence. Consistently, gel-shift experiments using a CpG-methylated, partially methylated, and hemimethylated o2 promoter fragments showed that, in vitro, the O2 protein binds to the major groove of a methylated target sequence, although its binding activity decreases at increasing levels of C-methylation and is more effectively reduced by methylation of the lower strand than of the upper strand of the DNA. Using partially purified endosperm cell extracts, we also show that, besides the O2 protein, other proteins specifically bind to a partially methylated o2 promoter fragment, therefore indicating that in plants a subset of different proteins may mediate the expression of a naturally occurring methylated o2 promoter.

Maize bronze 1:dSpm insertion mutations that are not fully suppressed by an active Spm

The Suppressor-mutator (Spm) family of maize transposable elements consists of autonomous Spm elements and nonautonomous defective Spm (dSpm) elements. One characteristic of this family is that the insertion of dSpm elements into a structural gene often permits some level of structural gene expression in the absence of SpM activity, and this structural gene expression is suppressed in trans by Spm activity. The Spm's subterminal repetitive regions (SRRs) contain several iterations of a 12-bp repeat motif. It had been proposed that binding of an Spm-encoded protein to these repeat motifs blocks structural gene transcriptional readthrough, thus suppressing gene expression. The bz-m13 allele of the bronze 1 locus contains a 2.24-kb dSpm insertion in the second exon of a Bz allele. In the absence of Spm activity, bz-m13 displays substantial Bz expression, and this expression is fully suppressed by Spm. Four intra-dSpm deletion derivatives are described in which this Bz expression is only partially suppressed by Spm. Each of these derivatives retains at least 12 SRR repeat motifs. Thus the presence of these repeat motifs is not sufficient to guarantee complete suppression by Spm. Some other property such as secondary structure or element size must play a role.