Biology Reference
In-Depth Information
Kumar, Saha, & Golden
2011
). However, consistent with its regulation by
NrrA (
Ehira & Ohmori, 2011
),
sigE
could also have a function in vegetative
cells in the absence of combined nitrogen. Further studies, which should
overcome the complications derived from functional redundancy, will be
needed to understand the role played by different sigma factors in the regu-
lation of heterocyst differentiation.
4.4. Regulators Impacting the Pattern of Heterocyst
Distribution
4.4.1. PatS and HetN
As mentioned above, heterocysts are nonrandomly distributed in the cyano-
bacterial filament and follow a periodic pattern that in strains of the genera
Anabaena
and
Nostoc
consists of a succession of heterocysts separated by c.
10-15 vegetative cells. Several regulators have been identified as involved
in heterocyst pattern formation. The
patS
gene is activated early during
heterocyst differentiation, and its inactivation produces a 'multiple contig-
uous heterocysts' (Mch) phenotype, whereas its overexpression abolishes
differentiation (
Yoon & Golden, 1998
). This gene encodes a polypeptide
whose C terminus consists of the pentapeptide RGSGR, known as PatS-5,
which when added to the external medium inhibits differentiation but fails
to restore a normal heterocyst pattern in a
patS
mutant (
Yoon & Golden,
1998
). It has been suggested that PatS, or a derivative of it, is exported from
the differentiating cells to build a gradient of an inhibitory signal that pre-
vents the differentiation of its neighbours. Genetic evidence (
Khudyakov &
Golden, 2004
) and in vitro studies (
Du et al., 2012
;
Feldmann et al., 2011
,
2012
;
Huang et al., 2004
;
Risser & Callahan, 2007
) have shown interaction
of PatS (or PatS subsets) with HetR, resulting in inhibition of HetR bind-
ing to DNA. In
patS
mutants, the Mch phenotype is seen in the first round
of heterocyst differentiation after nitrogen stepdown, but it is alleviated later,
consistent with a decrease in the expression of this gene.
The
hetN
gene, whose product has putative ketoacyl reductase motifs
and includes the RGSGR (PatS-5) pentapeptide, is activated late during
differentiation and, as is the case for
patS
, its inactivation produces a Mch
phenotype, although this phenotype is expressed at later times than in
patS
mutants. Overexpression of
hetN
also suppresses heterocyst differentiation
(
Black & Wolk, 1994
;
Callahan & Buikema, 2001
). Whereas conflicting
results have been published concerning the role of the reductase activity, the
RGSGR sequence appears to be required for the negative effect of HetN
on heterocyst differentiation (
Higa et al., 2012
;
Liu & Chen, 2009
). Thus,
