24 P. Doerner
presence of clusters ofcis-elements termed the OCTAMER (OCT) element
and the HEXAMER (HEX) element, respectively. Both of these were shown
to be necessary for S-phase periodic expression (Ohtsubo et al. 1997), and
as these binding sites are usually in close proximity, it is possible they inter-
act in a complex. The factors binding to HEX elements have been identified
and belong to the basic–leucine zipper (bZIP) class of transcription factors
(Tabata et al. 1989, 1991). However, bZIP factors define a large gene family,
and the specific members of this family involved in periodic, S-phase specific
gene expression of histones and other genes in vivo have yet to be identified. It
is possible that some HEX-binding factors have a more auxiliary role in sup-
porting, but not mediating, periodic gene expression: the distribution of HEX
and closely-related motifs is much more widespread, while OCT elements are
present in fewer genes, albeit not restricted to histones.
Gel shift analysis suggests that nuclear proteins from wheat binding to the
OCT element and HEX binding protein 1a (HBP-1a) might mediate periodic
S-phase RNA accumulation: the formation of S-phase specific complexes was
observed for one of three OCT-specific complexes and for one of three HBP-
1a-type complexes detected, respectively (Minami et al. 2000). However, the
genes encoding these factors have not yet been identified, and therefore, it is
still not clear whether these factors have a broader role in mediating S-phase
periodic transcription or whether it is restricted to one aspect of the S-phase
gene expression program.
3.2
E2F/DP/Rb
E2F transcription factors were first discovered in animal cell extracts as
positive regulators of S-phase gene expression that interact with the tumor
suppressor gene product of the Retinoblastoma locus (pRb) (Mudryj et al.
1990; Chellappan et al. 1991). The interaction with the Rb protein inhibits
E2F transcriptional activity. To bind to their target promoters, most E2F gene
products interact with a co-activator DP that shares a homologous DNA bind-
ing domain, although recently E2F-type transcription factors were identified
in plants and animals that have two DNA binding domains and can bind their
targets autonomously. E2F proteins mediate the expression of S-phase genes,
and ectopic expression of activating E2F proteins is sufficient to force S-phase
specific gene expression; but E2F proteins are also involved in regulating the
transition to cell differentiation and keeping stem cells in an undifferentiated
state. E2F activity is regulated through the relief of pRb-imposed inhibition,
which is mediated by CDK-dependent hyperphosphorylation. The specific
functions of E2F proteins in controlling cell cycle progression are still con-
tentious (Attwooll et al. 2004), but they are undoubtedly effectors of cell cycle
gene expression networks. For a more detailed discussion of plant E2F genes
and their function, see the chapter by Prof. Gutierrez in this volume.