but the gene remains expressed. Further more detailed methylation experiments show that methylation
of the transcriptional control region of the gene leads to gene inactivation but methylation of the body of
the gene does not inhibit gene expression. Thus, at least for some RNA polymerase II-transcribed genes,
demethylation of promoter regions is necessary for transcriptional activation but methylation of internal
regions appears to be unimportant.
Intriguingly, there seems to be a link between DNA methylation and histone deacetylation (see above).
One of the proteins found in the histone deacetylation polyprotein complex is Me-CpG binding protein.
This implies that DNA methylation controls gene expression by inducing histone deacetylation and con-
sequently chromatin compaction.
6.6.3.3 Transcriptional Initiation for RNA Polymerases I and III. RNA polymerase I transcribes a
single type of gene, namely the rRNA gene cluster. Upstream regions of such gene clusters are important in
their gene expression but the sequences have diverged so much between organisms that we cannot easily
identify ‘homology boxes’. This may be due to the highly repetitious nature of the rRNA gene cluster.
Transcriptional initiation for RNA polymerase I, involves protein components of similar complexity to
that for RNA polymerase II. However, the RNA polymerase I promoter is simpler and contains a single
type of upstream control element as well as the core promoter region surrounding the transcription start
site. A dimeric transcription factor called upstream binding factor(UBF), together with at least three other
factors, binds to both regions.
For most genes transcribed by RNA polymerase III no conserved upstream regions are discernible.
Instead the transcriptional control elements for most RNA polymerase III genes reside unusually within
the genes themselves.^45 Transcription factors TFIIIA (4 104 Da) and TFIIIC bind to these internal pro-
moters (Figure 6.24). The binding of these two factors is required for binding of a third factor, TFIIIB,
upstream of the start site. It is TFIIIB that aids RNA polymerase III binding.
6.6.4 DNA Replication
6.6.4. 1Introduction. Before a cell divides it must have already created an exactly duplicated set of
chromosomes so that both daughter cells can carry a set of genes identical to those in the parental cell. The
basis for this DNA replication is carried within the DNA itself. First, the DNA double helix carries two com-
plementary copies of the genetic information encoded within it (one on each strand). Secondly, Watson–Crick
Genes and Genomes 231
Figure 6.23 DNA methylation and gene expression