Frequent silencing of rDNA loci on the univalent-forming genomes contrasts with their stable expression on the bivalent-forming genomes in polyploid dogroses (Rosa sect. Caninae)

The polyploid species in Rosa section Caninae (2n=21, 28 or 35) are characterized by an unusual reproductive system known as odd (or asymmetric) meiosis. Only two chromosome sets form bivalents in meiosis, whereas the remaining chromosomes are transmitted as univalents through the female germline. Evolution of ribosomal rRNA genes (rDNA) does not seem to be significantly affected by interlocus homogenization in dogroses. As a consequence, most species contain several rDNA families falling into two main clades (beta and gamma) thought to be differentially distributed between bivalent and univalent chromosomes, respectively. Here, we have investigated expression of rRNA gene families in five pentaploid species (R. canina, R. rubiginosa, R. dumalis, R. sherardii and R. caesia, 2n=35) and in one tetraploid (R. mollis, 2n=28). Using extensive sequencing of ITS clones and cleaved amplified polymorphism sequence (CAPS) analysis, we found that the beta-family was constitutively expressed in all species. However, there was large variation in the expression patterns of families constituting the gamma-clade. In addition, a single family can be active in one species, whereas silenced in another. The data show that the families on bivalent-forming chromosomes dominate rDNA expression in all dogrose species. We hypothesize that genes on bivalent genomes are stably expressed, whereas those on univalent genomes undergo variable levels of epigenetic silencing. Nonetheless, mosaic expression of univalent genomes could contribute to phenotypic variation between the species.

The origin and evolution of geminivirus-related DNA sequences in Nicotiana

A horizontal transmission of a geminiviral DNA sequence, into the germ line of an ancestral Nicotiana, gave rise to multiple repeats of geminivirus-related DNA, GRD, in the genome. We follow GRD evolution in Nicotiana tabacum (tobacco), an allotetraploid, and its diploid relatives, and show GRDs are derived from begomoviruses. GRDs occur in two families: the GRD5 family's ancestor integrated into the common ancestor of three diploid species, Nicotiana kawakamii, Nicotiana tomentosa and Nicotiana tomentosiformis, on homeologous group 4 chromosomes. The GRD3 family was acquired more recently on chromosome 2 in a lineage of N. tomentosiformis, the paternal ancestor of tobacco. Both GRD families include individual members that are methylated and diverged. Using relative rates of synonymous and nonsynonymous nucleotide substitutions, we tested for evidence of selection on GRD units and found none within the GRD3 and GRD5 families. However, the substitutions between GRD3 and GRD5 do show a significant excess of synonymous changes, suggesting purifying selection and hence a period of autonomous evolution between GRD3 and GRD5 integration. We observe in the GRD3 family, features of Helitrons, a major new class of putative rolling-circle replicating eukaryotic transposon, not found in the GRD5 family or geminiviruses. We speculate that the second integration event, resulting in the GRD3 family, involved a free-living geminivirus, a Helitron and perhaps also GRD5. Thus our data point towards recurrent dynamic interplay between geminivirus and plant DNA in evolution.

Distribution of 5-methylcytosine residues in 5S rRNA genes in Arabidopsis thaliana and Secale cereale

Bisulfite genomic sequencing was used to localise 5-methylcytosine residues (mC) in 5S rRNA genes of Arabidopsis thaliana and Secale cereale. The maps of mC distribution were compared with the previously published map of the corresponding region in Nicotiana tabacum. In all three species, the level of methylation of 5S rRNA genes was generally higher than the average for the entire genome. The ratio of 5S rDNA methylation to average overall methylation was 44%/30-33% for N. tabacum, 27%/4-6% for A. thaliana and 24%/20-22% for S. cereale. With the exception of one clone from S. cereale, no methylation-free 5S rDNA was detected. The level of methylation at different sequence motifs in 5S rDNA was calculated for N. tabacum/A. thaliana/ S. cereale, and this analysis yielded the following values (expressed as a percentage of total C): mCG 90%/78%/85%, mCWG 89%/41%/53%, mCmCG 72%/32%/16%, mCCG 4%/2%/0%, mCHH 15%/6%/1%, where W=A or T, and H=A or C or T. Non-symmetrical methylation was almost negligible in the large genome of S. cereale but relatively frequent in N. tabacum and A. thaliana, suggesting that the strict correlation between genome size and cytosine methylation might be violated for this type of methylation. Among non-symmetrical motifs the mCWA triplets were significantly over-represented in Arabidopsis, while in tobacco this preference was not as pronounced. The differences in methylation levels in different sequence contexts might be of phylogenetic significance, but further species in related and different taxa need to be studied before firm conclusions can be drawn.

Hypomethylation of CNG targets induced with dihydroxypropyladenine is rapidly reversed in the course of mitotic cell division in tobacco

We followed the mitotic transmission of an experimentally induced hypomethylated state of several tobacco repetitive sequences in callus culture and plants. The initial hypomethylation was induced by a hypomethylation drug, dihydroxypropyladenine (DHPA), the competitive inhibitor of cellular S-adenosylhomocysteine hydrolase, which is known to preferentially inhibit methylation at CNG and non-symmetrical motifs while having a negligible effect on methylation at CG motifs. The deprivation of this drug resulted in an almost immediate remethylation of cytosines at CNG motifs ( MspI and EcoRII sites) leading us to conclude that, the hypomethylation effect of dihydroxypropyladenine is rather transient and differs from that of 5-azacytidine which often induces heritable changes in methylation patterns. The results suggest that de novo methylation of CNG motifs is a rapid and meiotically independent process on DNA sequences with pre-existing CG methylation.

Evolution of 5S rDNA unit arrays in the plant genus Nicotiana (Solanaceae)

Nicotiana tabacum (tobacco, Solanaceae) has two 5S ribosomal DNA (rDNA) families, one of unit length approximately 646 bp and the other -430 bp, that differ in the length of the 5S rDNA non-transcribed spacer (NTS). The long 5S rDNA family, found on the T genome of tobacco and in Nicotiana tomentosiformis, contains a GC-rich subregion that is absent in the short family. We designed primers for this subregion and generated a probe that we used against a range of Nicotiana and related Solanaceous species. We demonstrated the presence of the GC-rich subregion in a range of Nicotiana species, but it was absent in Nicotiana sylvestris, Nicotiana longiflora, and two closely related genera, Petunia and Solanum. These data suggest that this subregion of the NTS is likely to have evolved with the genus Nicotiana. The absence of the subregion in N. sylvestris and N. longiflora is likely to have arisen by a deletion event in the evolution of section alatae. We demonstrate patterns of evolution in the 5S rDNA unit cluster in relation to a phylogenetic reconstruction of species relationships in section tomentosae. Nicotiana glutinosa diverged early from the section and contains a 5S rDNA family based on a 550-bp unit. After this divergence, 430- and 650-bp rDNA unit families evolved. The 650-bp family is found in all species of tomentosae (except N. glutinosa) and in tobacco. The 430-bp family within tomentosae includes the GC-rich subregion and is thus unrelated to the 430-bp family in N. sylvestris. Nicotiana setchellii is unusual in that it has three 5S rDNA loci, including one locus that is exceptionally large. This species, unique to tomentosae, has a very abundant 900-bp unit family. It is possible that this 900-bp family occurs on this one large locus. In N. tomentosa and N. kawakamii, the 650-bp family is predominant, whereas N. tomentosiformis and N. otophora have only the 650-bp family. There is no clear relationship between the number of 5S families and the number of 5S rDNA loci. Certainly the replacement of 5S rDNA units, perhaps by gene conversion, has occurred repeatedly in the evolution of genus Nicotiana.

Evolution and structure of 5S rDNA loci in allotetraploid Nicotiana tabacum and its putative parental species

Nicotiana tabacum (tobacco) is an allotetraploid derived from ancestors of the modern diploids, N. sylvestris and N. tomentosiformis. We identified and characterized two distinct families of 5S ribosomal DNA (rDNA) in N. tabacum; one family had an average 431 bp unit length and the other a 646 bp unit length. In the diploid species, N. sylvestris and N. tomentosiformis, the 5S rDNA unit lengths are 431 bp and 644 bp respectively. The non-coding spacer sequence of the short unit in tobacco had high sequence homology to the spacer of N. sylvestris5S rDNA, while the longer spacer of tobacco had high homology with the 5S spacer of N. tomentosiformis. This suggests that the two 5S families in tobacco have their origin in the diploid ancestors. The longer spacer sequence had a GC rich sub-region (called the T-genome sub-region) that was absent in the short spacer. Pulsed field gel analysis and fluorescent in situ hybridization to tobacco metaphase chromosomes showed that the two families of 5S rDNA units are spatially separate at two chromosomal loci, on chromosomes S8 (short family) and T8 (long family). The repeat copy number at each chromosomal locus showed heterogeneity between different tobacco cultivars, with a tendency for a decrease in the copy number of one family to be compensated by an increase in the copy number of the second family. Sequence analysis reveals there is as much diversity in 5S family units within the diploid species as there is within the T and S-genome 5S family units respectively, suggesting 5S diversification within each family had occurred before tobacco speciation. There is no evidence of interlocus homogenization of the two 5S families in tobacco. This is therefore substantially different to 18-26S rDNA where interlocus gene conversion has substantially influenced most sequences of S and T genome origin; possible reasons are discussed.

Transposition of IS10 from the host Escherichia coli genome to a plasmid may lead to cloning artefacts

During recloning of Nicotiana tabacum L. repetitive sequence R8.3 in Escherichia coli, a modified clone that differed from the original by the insertion of an IS10 sequence was unintentionally produced. The insert was flanked by a 9-bp direct repeat derived from the R8.3 sequence, the 9-bp duplication of acceptor DNA in the site of insertion being a characteristic of IS10 transposition events. A database search using the FASTA program showed IS10 and other prokaryotic IS elements inserted into numerous eukaryotic clones. Unexpectedly, the IS10, which is not a natural component of the E. coli genome, appeared to be by far the most frequent contaminant of DNA databases among several IS sequences tested. In the GenEMBL database, the IS10 query sequence yielded positive scores with more than 500 eukaryotic clones. Insertions of shortened IS10 sequences having only one intact terminal inverted repeat were commonly found. Most full-length IS10 insertions (32 out of 40 analyzed) were flanked by 9-bp direct repeats having the consensus 5'-NPuCNN-NGPyN-3' with a strong preference for 5'-TGCTNA-GNN-3'. One insertion was flanked by an inverted repeat of more than 400 bp in length. PCR amplification and Southern analysis revealed the presence of IS10 sequences in E. coli strains commonly used for DNA cloning, including some reported to be Tn10-free. No IS10-specific PCR product was obtained with N. tabacum or human DNA. Our data suggest that transposition of IS10 elements may accompany cloning steps, particularly into large BAC vectors. This might lead to the relatively frequent contamination of DNA databases by this bacterial sequence. It is estimated that one in approximately every thousand eukaryotic clone in the databases is contaminated by IS-derived sequences. We recommend checking submitted sequences for the presence of IS10 and other IS elements. In addition, DNA databases should be corrected by removing contaminating IS sequences.

Cytosine methylation of plastid genome in higher plants. Fact or artefact?

DNA methylation of chloroplast genome has been studied in a large variety of angiosperm species using restriction enzyme analysis of three genomic loci (totally encompassing about 10% of chloroplast genome) and bisulfite genomic sequencing of tobacco ribulose bisphosphate carboxylase/oxygenase (large subunit) gene (rbcL). Except for CCWGG (W=A or T) sites that were partially refractory to the cleavage with methylation sensitive EcoRII in all loci, no cytosine methylation was found at the CCGG (MspI/HpaII) and several other restriction sites tested. However, EcoRII was unable to completely digest an unmethylated CCWGG site in the cloned rbcL gene on plasmid. Further a bisulfite genomic sequencing performed on EcoRII-restricted DNA failed to show any 5-methylcytosine either within or outside inspected EcoRII sites along the 3' end of rbcL coding region. In conclusion our results do not support evidence for methylated cytosine residues in plant chloroplast genomes and we suggest that results obtained with EcoRII should be interpreted with great care especially when small differences in methylation levels are analysed.

Drug-induced hypomethylation of a posttranscriptionally silenced transgene locus of tobacco leads to partial release of silencing

The effect of DNA methylation upon posttranscriptional gene silencing (PTGS) has been investigated in transgenic tobacco lines showing PTGS and methylation of the neomycin phosphotransferase II (nptII) reporter genes. Application of the hypomethylation drugs dihydroxypropyladenine or 5-azacytidine resulted in approximately 30% reduced methylation of cytosines located in a non-symmetrical context in the 3' untranslated region of the nptII transgenes. The hypomethylation was accompanied by up to 12-fold increase in NPTII protein levels, suggesting that methylation of non-symmetrical motifs may account for an increased degree of PTGS. Models for the possible role of DNA methylation in PTGS are discussed.

Cross-talk between posttranscriptionally silenced neomycin phosphotransferase II transgenes

Tobacco plants containing a transgene locus with two chimeric neomycin phosphotransferase II (nptII) genes in tail-to-tail orientation (locus 1) show posttranscriptional gene silencing. The silenced nptII transgenes of locus 1 can downregulate the expression of homologous nptII transgenes in hybrid plants. The 3' region of the silenced nptII genes located in the center of the inverted repeat locus 1 is extensively methylated. Moreover, 3' segments of in trans-inactivated transgenes also become methylated, suggesting cross-talk between homologous posttranscriptionally silenced genes. Our results are in accordance with the hypothesis that this cross-talk can be mediated by specially featured RNAs.

Comparative analysis of DNA methylation in tobacco heterochromatic sequences

Cytosine methylation levels and susceptibility to drug-induced hypomethylation have been studied in several Nicotiana tabacum (tobacco) DNA repetitive sequences. It has been shown using HapII, MspI, BamHI and Sau3AI methylation-sensitive restriction enzymes that the degree of 5'-mCmCG-3' methylation varied significantly between different repeats. There were almost saturation levels of 5-methylcytosine at the inner (3') cytosine position and variable degrees of methylation at the outer (5') cytosine at the enzyme recognition sites. The non-transcribed high copy satellite sequences (HRS60, GRS) displayed significant heterogeneity in methylation of their basic units while middle repetitive sequences (R8.1, GRD5, 5S rDNA) were more uniformly modified at both cytosine residues. Dihydroxypropyladenine (DHPA) treatment, which is thought to reduce DNA methyltransferase activity by increasing S-adenosylhomocysteine levels, resulted in extensive demethylation of the outer cytosine in all repeats, and the partial hypomethylation of cytosines at the inner positions in less densely methylated repeats such as HRS60 and GRS. The results suggest that hypomethylation of 5'-mCmCG-3' sites with DHPA is a gradual non-random process proceeding in the direction mCmCG-->CmCG-->CCG. The 18S-5.8S-25S rDNA was remarkably hypomethylated relative to the 5S rDNA at all restriction sites studied. Fluorescence in-situ hybridization showed that DNA decondensation within and between the 18S-5.8S-25S and 5S rDNA loci was variable in different nuclei. All nuclei had condensed and decondensed sequence. The chromatin of 18S-5.8S-25S rDNA was more readily digested with micrococcal nuclease than the 5S rDNA suggesting that the overall levels of decondensation were higher for 18S-5.8S-25S rDNA. Variable decondensation patterns within and between loci were also observed for GRS and HRS60. Cytosine methylation of the tobacco repeats is discussed with respect to transcription, overall levels of condensation and overall structure.

Efficacy of polymerase chain reaction for detection of deep mycotic infections: confirmation by autopsy

A method based on polymerase chain reaction (PCR) amplification of fungal 18S rDNA sequences was tested for the detection of fungi in blood samples. In order to increase sensitivity and specificity, PCR products were hybridized to the radioactively labelled fragment of 18S rDNA gene. Blood from 28 patients with haematological malignancies was taken immediately after death and the results of PCR analysis were compared with results of autopsy examination. To the best of our knowledge, no study of such a design has been published previously. PCR analysis turned out to be very sensitive (92%) and specific (92%) as well as capable of detecting various kinds of fungal infections (localized as well as generalized).

Mapping of 5-methylcytosine residues in Nicotiana tabacum 5S rRNA genes by genomic sequencing

Genomic sequencing was used to localise 5-methylcytosine residues in individual DNA strands of 5S rRNA genes in tobacco. The density of methylation along the sequence was high in both strands, exceeding the average methylation density of the tobacco genome. Besides methylation of CG and CNG sequences, considerable amounts of mC were found in non-symmetrical sites. Among 69 sequenced clones obtained from leaf DNA we did not detect any non-methylated clone, and Southern blot hybridisation analysis also failed to suggest the presence of methylation-free 5S rDNA units in the tobacco genome. Differences were observed among methylation patterns of individual sequenced clones. This heterogeneity reflects either heterogeneity among individual members of 5S rRNA gene cluster or differences among individual cells. Methylation of CNG and non-symmetrical sites can be efficiently reduced by treatment with dihydroxypropyladenine, an inhibitor of S-adenosylhomocysteine hydrolase.

Evaluation of the impact of S-adenosylhomocysteine metabolic pools on cytosine methylation of the tobacco genome

We explored the possibility that the cytosine DNA methylation might be regulated by S-adenosyl-L-methionine (AdoMet) and S-adenosyl-L-homocysteine (AdoHcy) pools in plant cells. In order to change the AdoHcy/AdoMet ratio (methylation index), (S)-9-(2,3-dihydroxypropyl)adenine was employed, a selective reversible inhibitor of cellular S-adenosyl-L-homocysteine hydrolase. Micromolar concentrations of the inhibitor increased dramatically (more than 1000-fold) intracellular AdoHcy levels (and concominantly the AdoHcy/AdoMet ratio) in tobacco TBY-2 cells. No toxic effect of the drug was observed and the cells displayed only marginal inhibition of growth at high AdoHcy levels. At near equal intracellular concentrations of AdoHcy and AdoMet, a significant reduction of cytosine methylation in transcribed (5SrDNA) and non-transcribed (HRS60, NTRS) sequences was observed. Interestingly, the CpCpG and CpApG trinucleotide targets appeared to be most sensitive to changes in the methylation index. Methylation of cytosine residues at CpG sites was not affected even at AdoHcy/AdoMet ratio of > 10. These results support the possible regulation of DNA methylation via AdoHcy/AdoMet metabolic pathways in plant cells.

Variability in CpNpG methylation in higher plant genomes

The methylation status of ribosomal gene (rRNA) clusters have been investigated in a large variety of angiosperm species. Here we have analysed methylation in ribosomal gene (rRNA) clusters using MspI, HpaII, BstNI, EcoRII and CfoI restriction enzymes in combination with Southern hybridization to the 25S rDNA probe. It was shown that cytosine methylation at CpG dinucleotides and CpNpG trinucleotides occurred in all plant genomes examined. Methylation of rDNA units at CpG dinucleotides (studied with CfoI) was high in all species tested with approx. 40-70% of units being completely or nearly completely methylated. In contrast, the extent of the CpNpG methylation (studied with MspI and EcoRII) varied significantly between species; the percentage of the rDNA fraction entirely methylated at CpNpG trinucleotides ranged from less than 1% to almost 90% depending on the genome studied. Larger interspecies than within species variation was also observed among several non-transcribing repetitive sequences. In a small genome of A. thaliana, the CpNpG methylation appeared to be highly compartmentalized into the repetitive fraction. The methylation of trinucleotides was abundant in large A+T-rich genomes and it is proposed that the CpA(T)pG trinucleotides may help to maintain a high density of methylatable targets in plant repeated sequences.

Chromatin fragmentation associated with apoptotic changes in tobacco cells exposed to cold stress

Programmed cell death (PCD) may be triggered by a variety of environmental stimuli. In this report we show that low temperature treatment of tobacco BY-2 cells results in specific chromatin changes. The early stage was characterised by chromatin condensation associated with specific endonucleolytic cleavage of the genome into fragments of 50-100 kbp in size. Later, after 2 weeks of the cold treatment, a ladder of nucleosomal units (178 bp) and their multiples occurred. Chromatin changes were accompanied by a general decrease in cell viability. However, the cell culture retained about 11% of living cells even after prolonged incubation in the cold suggesting the presence of a cold-resistant population of cells. The results support the view that PCD was activated by the cold stress. We suggest that cold-stressed tobacco BY-2 culture might be a useful system for investigation of PCD in plant cells.

[Diagnosis of mycotic infections in oncology patients using the polymerase chain reaction]

Diagnosis of mycotic infections is despite the immense effort devoted to this problem still very inaccurate. A new and promising method is the polymerase chain reaction, PCR, which theoretically can detect a single cell. In their original study the authors decided to develop a new method for the detection of fungi by PCR and to compare this examination with post-mortem findings. Thus it was possible to determine sufficiently reliably the sensitivity and specificity of the method. For the detection of fungi the authors selected the sequence coding for a small subunit of ribosomal RNA (18S rDNA). The method is able to detect the amount of DNA from some 10-100 cells. The sensitivity was 90% and the specificity 92%. The method is so far too laborious for common practice, its simplification would be however very useful.

Telomerase activity in plant cells

Telomerase is a ribonucleoprotein enzyme which elongates the G-rich strand of telomeric DNA to compensate for the progressive reduction in its length due to incomplete replication of chromosome ends, which in human somatic cells leads to cell cycle arrest upon shortening of telomeres to a critical length. To examine the possible involvement of telomerase in metabolism of plant genetic material, we used cells of Nicotiana tabacum strain TBY-2, a stable long-term culture which has kept a constant pattern of restriction fragments from chromosome termini during its 6 month period of cultivation in our laboratory. In a direct assay for telomerase, a 5' end-labeled plant telomeric oligonucleotide 5' (TTTAGGG)(3')6 was elongated in a TBY-2 cell extract, showing a pausing pattern which is a characteristic feature of telomerases from other organisms. The elongation was inhibited by RNase A pretreatment of the extract. We conclude that plant cells possess telomerase which is used for maintenance of their telomeres.

Organization of telomeric and subtelomeric chromatin in the higher plant Nicotiana tabacum

We have examined the structure and chromatin organization of telomeres in Nicotiana tabacum. In tobacco the blocks of simple telomeric repeats (TT-TAGGG)n are many times larger than in other plants, e.g., Arabidopsis thaliana or tomato. They are resolved as multiple fragments 60-160 kb in size (in most cases 90-130 kb) on pulsed-field gel electrophoresis (PFGE) of restriction endonuclease-digested DNA. The major subtelomeric repeat of the HRS60 family forms large homogeneous blocks of a basic 180 bp motif having comparable lengths. Micrococcal nuclease (MNase) cleaves tobacco telomeric chromatin into subunits with a short repeat length of 157 +/- 5 bp; the subtelomeric heterochromatin characterized by tandemly repeated sequences of the HRS60 family is cut by MNase with a 180 bp periodicity. The monomeric and dimeric particles of telomeric and subtelomeric chromatin differ in sensitivity to MNase treatment: the telomeric particles are readily digested, producing ladders with a periodicity of 7 bp, while the subtelomeric particles appear to be rather resistant to intranucleosomal cleavage. The results presented show apparent similarities in the organization of telomeric chromatin in higher plants and mammals.

The telomeric sequence is directly attached to the HRS60 subtelomeric tandem repeat in tobacco chromosomes

PCR and primers derived from the telomeric repeat (CCCTAAA)n and from the tobacco subtelomeric tandemly repetitive sequence HRS60 (EMBL X12489) were used to amplify the region linking the two loci. A 131 bp PCR product was obtained both from total tobacco DNA and from the DNA fraction enriched for telomeres. Its sequence only consists of the telomeric primer and the attached region of the HRS60 repetitive unit up to the end of the sequence complementary to the HRS60 primer. The site of direct continuity between the two sequences is formed by a (dA)7 tract.

DNA curvature of the tobacco GRS repetitive sequence family and its relation to nucleosome positioning

Recently, a highly repetitive DNA sequence family (GRS) from tobacco was described in our laboratory. These sequences were found to be localized predominantly in the pericentromeric heterochromatin of tobacco chromosomes. To test the hypothesis that these sequences play an important role in the formation of heterochromatin, we investigated the DNA curvature of the GRS sequences and its possible impact to the chromatin structure at these loci. Application of the nearest-neighbour wedge model of intrinsic DNA curvature for the GRS1 family member predicted two loci of curvature: a major bend at the 5' end of the sequence and a minor bend of opposite direction at the centre of the GRS1. The presence of the major and the minor loci of DNA curvature was studied experimentally using permutation analysis and site-directed mutagenesis. The experimental results were consistent with the computer predictions. We gave evidence that the described DNA curvature is also present in the entire GRS family. Genomic statistical sequencing showed the conservation of the major bend sequence determinants in the members of the GRS family. To investigate the chromatin structure at the GRS sequences, we determined the nucleosome positioning in vivo at these sequences using thermal cycle primer extension. A relation between the curvature pattern and the histone octamer position was observed: the major bend is excluded from the nucleosome surface to the linker region, while the minor bend is distributed along the core DNA. The suggestion is made that the sequences in the minor locus of curvature define the rotational setting of the nucleosome, and a possible role of the major bend as a factor, which defines the translational setting, is discussed.

Sequence-specific hypomethylation of the tobacco genome induced with dihydroxypropyladenine, ethionine and 5-azacytidine

Higher plant DNA is methylated at CG and CNG targets. In this study we have investigated the tobacco methylation system in tissue culture using the methylation inhibitors 5-azacytidine (5-azaC), dihydroxypropyladenine (DHPA) and ethionine (Ethi), and methylation-sensitive restriction endonucleases HpaII, MspI, HhaI, EcoRII, ScrFI, and Fnu4HI. Surprisingly, CAG/CTG sequences, contrary to CG doublets and CCG/CGG triplets, appeared to be refractory to the inhibitory effect of 5-azaC. Thus 5-azaC cannot be considered a general inhibitor of DNA methylation in tobacco cells. On the other hand, DHPA, the inhibitor of S-adenosylhomocysteine (SAH) hydrolase, and Ethi caused hypomethylation of both CAG/CTG and CCG/CGG triplets but not of the CG doublets. The sensitivity of triplet-specific methylation to the inhibition of SAH hydrolase suggests the possibility that plant-specific DNA methylation at CNG targets might be modulated by alterations of the SAH/S-Adenosylmethionine ratio in plant cells.

Immunological quantification of RecA protein in cell extracts of E. coli after exposure to chemical mutagens or UV radiation

Increased synthesis of RecA protein is induced in E. coli cells after their damage, the rate of synthesis being dependent on the extent of DNA alterations. The level of the RecA protein was determined in E. coli cell extracts after damage induced by NQO, MNNG, MMC, NAL or UV radiation, using competitive enzyme-linked immunosorbent assay (ELISA). Purified E. coli RecA protein and rabbit monospecific polyclonal antibodies against it were prepared for the quantitative assay. The level of the RecA protein was increased after treatment with all mutagens. Contrary to other induced proteins, the synthesis of the RecA protein increased within 30 min after damage with UV radiation at a relatively slow rate. The ELISA method made it possible to determine 0.5-50 ng of the RecA protein in bacterial extracts. The method can be employed as an auxiliary test for DNA damage determination and also in studied concerning the role of the RecA protein in repair processes.

No significant correlation between specific antibodies to mouse mammary tumour virus and human cancer

To study the possible involvement of mouse mammary tumour virus (MMTV) related agent in human cancer we analysed 300 samples of human sera for the presence of antibodies to MMTV structural proteins. All sera were tested by immunoblotting to achieve high specificity. Out of 300 sera, 22 reacted with transframe protein p30, 16 with the ribonucleoprotein p14, six with the envelope glycoprotein gp52 and three with the major core protein p27. Reactivities to p30 and p14 were observed in sera from cancer patients and healthy controls; reactivities to p27 and gp52 predominated in sera of cancer patients. Sera frequently reacted with a 42 kDa protein which is a cellular contaminant of the virus.