Biology Reference
In-Depth Information
This again stresses the importance of deciphering the post-translational
regulation of HetR for the understanding of molecular mechanisms sup-
porting heterocyst differentiation.
The Ca
2+
-binding protein CcbP has been shown to be required for
formation of a normal pattern of heterocysts in
Anabaena
sp. strain PCC
7120 (
Zhao et al., 2005
). An
Anabaena
mutant lacking CcbP shows an Mch
phenotype, and a mutant overexpressing CcbP does not form heterocysts.
It has been suggested that CcbP sequesters Ca
2+
, which would be liber-
ated upon CcbP degradation in cells that are differentiating into heterocysts
(
Shi, Zhao, Zhang,Ye, & Zhao, 2006
), and this effect has been related to the
observation that nitrogen deficiency elicits an increase in the cellular levels
of calcium (
Torrecilla, Leganés, Bonilla, & Fernández-Piñas, 2004
).
The
patU3
gene is expressed in proheterocysts at late differentiation
times, and its inactivation produces an Mch phenotype (
Zhang et al.,
2007
). The
hetZ
gene, upregulated in proheterocysts, encodes a putative
DNA-binding protein with a helix-turn-helix motif, and its inactiva-
tion provokes delayed or no differentiation while impairing the induction
of some heterocyst differentiation genes such as
patS
,
hetC
and
cox2
, and
altering the patterned induction of
hetR
(
Zhang et al., 2007
). Thus, PatU3
and HetZ have been proposed to influence the co-ordination between
heterocyst differentiation and pattern formation (
Zhang et al., 2007
; see
also
Meeks et al., 2002
).
5. CONCLUSIONS AND PERSPECTIVES
We have described above the patterns of regulation of different types
of genes that are expressed during the process of heterocyst differentia-
tion, and have summarized some aspects of their regulation. A salient fea-
ture is the conspicuous role that the NtcA transcription factor plays at the
beginning of differentiation, during the differentiation process and in the
mature heterocyst (
Herrero et al., 2004
). The molecular basis for this role of
NtcA is the presence of NtcA-dependent promoters in many heterocyst-
related genes. But HetR, which can now be denoted a transcription factor
as well (
Kim et al., 2011
), is also needed for activation of transcription of
a number of these genes. The mechanism by which HetR exerts its role is
unknown, but the first hint is that it could be a co-activator of transcrip-
tion at some NtcA-dependent promoters (
Camargo et al., 2012
), consistent
with the fact that all HetR-dependent promoters are also dependent on
NtcA (although indirect effects of NtcA cannot be ruled out). As is the case
