Session 11: Chromosome Structure and Function.
Chair: Liz Dennis, CSIRO Division of Plant Industry, Canberra 2601, Australia
email: liz@pi.csiro.au
The session on Chromosome Structure and Function included talks on DNA methylation and gene silencing, on transcription factors and on meiotic mutants. The way in which methylation and transcription factors may interact in controlling gene expression gives us a new view of the different ways in which plant processes are controlled. There was also a related epigenetics workshop that produced useful discussion. Eric Richards is organising an email network to continue with the discussion in this exciting area.
In the session, Liz Dennis opened with a general overview of the connection between DNA methylation and the loss of gene activity as was shown early by the studies of the cycling Acs of maize. She also described more recent work showing that plants with reduced levels of methylation (either from decreased DNA methylation mutants or transgenic plants carrying antisense to DNA methyl.transferase genes), showed phenotypic abnormalities suggesting that disrupting DNA methylation patterns leads to alterations in development.
Herve Vaucheret described mutants impaired in post transcriptional transgene silencing and co-suppression. Two loci, sgs 1 & sgs 2 (suppressor of gene silencing) were defined - when 35S-Nia2 was introduced into the sgs mutants - co-suppression of host Nia genes was not triggered (0/8), whereas silencing occurred in 100% of cases when the construct was introduced into wild type plants. The sgs 1 & 2 mutations can also protect against the silencing of a 35S x 2-Uid gene.
Ortrun Mittelsten-Scheid spoke about release of epigenetic silencing by transacting mutations. She described selecting for a mutation that relieved silencing of a hygromycin resistance gene. These mutations were second site mutations (not in the hygromycin gene) and were transmitted to progeny. The hygromycin silencing was released by backcrossing to ddm1 mutants but not to antisense methyl transferase mutants.
Jeff Jeddeloh described his work on combining a ddm1-2 mutation with a silenced PA12 gene complex. The ddm mutation affects gene silencing and methylation immediately and progressively suggesting a model for the effects seen in ddm1 inbred lines where phenotypes can progess over generations and appear cumulative.
Steve Jacobsen presented his data from the novel "Clark Kent" mutants, which as might be expected, are alleles of SUPERMAN. Genomic sequencing showed these alleles result from hypermethylation of the SUPERMAN gene and associated lack of SUPERMAN transcript. This change in methylation status (some 5% of cytosines are methylated in the mutant) can be reversed and wild type " revertants " appear. Similar changes occur in the superman alleles seen in the antisense methyl transferase lines. The question of a normal role of methylation in the control SUPERMAN expression was discussed.
The importance of interactions between DNA binding proteins was emphasized by Karan Singh who discussed his findings with the octopine sythase element binding proteins (OBFS). He talked about DNA binding proteins that enhance the binding of other transcription factors such as proteins that contain Dof domains, which interact with B ZIP binding factors.
Another class of transcription factors which bind to OBFs include AtERB (ethylene responsive binding protein), which is known to bind to the GCC box (others in this class are CBF1, AP2 and TINY), but also increases binding of G box binding factors to the G box.
Finally, Chris Makaroff discussed his characterization of syn1 a synaptic mutant. He presented beautiful pictures of meiosis of both wild type and mutant plants which suggest synapsis was defective and unpaired chromosomes persist through the second meiotic division. This technique will also be important in looking at other mutants altered in meiosis.