Gene transfer in plant protoplasts Inhibition of gene activity by cytosine methylation and expression of single-stranded DNA constructs

Transient gene expression of foreign genes in preheated protoplasts: stimulation of expression of transfected genes lacking heat shock elements

Transfection of preheated petunia protoplasts with several biologically active DNA constructs resulted in a significantly higher gene expression than that observed in transfected unheated protoplasts. It was observed with supercoiled, linearized and single-stranded DNA structures that stimulation of transient gene expression in preheated protoplasts was neither dependent on the reporter gene nor on the regulatory elements used. Heat treatment at 42 degrees C also increased expression in protoplasts transfected with a plasmid bearing the tobacco mosaic virus (TMV) translational enhancer, omega. Northern blot analysis revealed that heat treatment of protoplasts before the transfection event greatly increased the amount of the newly synthesized transcripts. Preheating of protoplasts did not affect the transfection efficiency, namely the number of transfected cells in the population, nor the amount of DNA in transfected nuclei, as was inferred from histochemical staining and Southern blot analysis, respectively. The possible mechanism by which heat treatment stimulates transient gene expression of genes lacking obvious heat shock elements is offered. The relevance of the present findings to transient gene expression in plants in general and to viral gene expression in particular is discussed.

Transcriptional and translational control of gene expression in cauliflower mosaic virus

Cauliflower mosaic virus sequences have developed as a powerful tool for the study of various aspects of gene expression in plants. Analysis of the promoter/enhancer region has led to the discovery of several transcription factors and factor-binding sites. Studies on RNA processing and polyadenylation reveal a viral strategy to obtain terminal redundancy of retrovirus pregenomic RNA. Striking differences between plant and vertebrate polyadenylation signals have been disclosed. The mechanisms for translation of the polycistronic 35S RNA are novel in the eukaryotic field and may give new insight to translational control in general.

CpG methylation inhibits binding of several sequence-specific DNA-binding proteins from pea, wheat, soybean and cauliflower

To elucidate how methylation of specific sites in plant DNA might control transcription, we examined the effect of DNA methylation at CpG sequences on the binding of plant nuclear factors to an oligonucleotide duplex containing the consensus sequence for mammalian CREB (cAMP response element binding protein). CREB is part of the ATF (activating transcription factor) family of mammalian proteins specifically binding to 5'-TGACGTCA-3' and related sequences. Proteins recognizing the CREB-specific ligand were identified in nuclear extracts of pea seeds, wheat germ, cauliflower, and soybean leaves using electrophoretic mobility shift assays. Cytosine methylation inhibited binding of this protein in all these extracts, and so this sequence-specific DNA-binding activity is referred to as methylation-inhibited binding protein 1 (MIB-1). Sites somewhat similar to that of the CREB ligand are found in the upstream regions of a wheat histone H3 gene and tomato and pea ribulose 1,5-bisphosphate carboxylase genes. These sites were bound preferentially by distinct proteins that may be related to the previously described plant proteins HBP-1, HSBF, ASF-1, or GBF. Methylation of cytosine residues at these sites and at a site for MIB-1 located upstream of a soybean proline-rich protein gene also reduced specific binding with all the nuclear extracts tested. Similarly, substitution of the central CpG dinucleotide with TpG decreased binding.

Cytosine methylation in gene-silencing mechanisms

Cytosine methylation is associated with gene-silencing mechanisms in a number of eukaryotic organisms. Recent studies directed at the involvement of methylation in promoter inactivation, X-chromosome and duplicate sequence inactivation and in chromatin structure changes, are presented.

Modulation of DNA methylation and gene expression in cultured sycamore cells treated by hypomethylating base analog

The selective suppression of photosynthetic genes in both the nuclear and plastid genomes of the nonphotosynthetic white wild-type cell line of sycamore (Acer pseudoplatanus) has been found to be inversely related to the presence of a variety of methylated bases, especially 5-methylcytosine (5-MeCyt) and N6-methyladenine (N6-MeAde), localized in regions of the plastid genome containing silent genes. We used hypomethylating base analogs to manipulate the level of cytosine and adenine methylation in the white cells of sycamore, and examined the effects of changes in methylation on gene expression. Treatment with 5-azacytidine (5-AzaCyd) and N6-benzyladenine (N6-BzlAde) decreased cytosine and adenine methylation. This was accompanied by restoration of transcriptional activity in photosynthetic genes which are usually suppressed. Both 5-MeCyt and N6-MeAde suppressed nuclear gene expression, but only 5-MeCyt suppressed plastid gene expression.

Effect of CpG methylation on gene expression in transfected plant protoplasts

Activity of the cat gene driven by the cauliflower mosaic virus 35S promoter has been assayed by transfecting petunia protoplasts with the pUC8CaMVCAT plasmid. In vitro methylation of this plasmid with M.HpaII (methylates C in CCGG sites) and M.HhaI (methylates GCGC sites) did not affect bacterial chloramphenicol acetyltransferase (CAT) activity. It should be noted, however, that no HpaII or HhaI sites are present in the promoter sequence. In contrast, in vitro methylation of the plasmid with the spiroplasma methylase M.SssI, which methylates all CpG sites, resulted in complete inhibition of CAT activity. The promoter sequence contains 16 CpG sites and 13 CpNpG sites that are known to be methylation sites in plant DNA. In the light of this fact, and considering the results of the experiments presented here, we conclude that methylation at all CpG sites leaving CpNpG sites unmethylated is sufficient to block gene activity in a plant cell. Methylation of CpNpG sites in plant cells may, therefore, play a role other than gene silencing.