Gene Expression during Heterocyst Differentiation - Advances in Botanical Research - page 303

the absence of combined nitrogen (bubbled cultures enriched with CO 2 )
both in the wild-type strain and in a hetR mutant that is unable to dif-
ferentiate heterocysts ( Buikema & Haselkorn, 1991 ). They identified >900
putative TSPs whose use increased in response to nitrogen deficiency, of
which 209 were not induced in the hetR mutant suggesting that they are
involved in heterocyst differentiation. The TSPs that were independent
of HetR include some corresponding to genes involved in the assimila-
tion of sources of nitrogen alternative to ammonium and in scavenging of
traces of ammonium, as well as of genes involved in the general response
to nitrogen deprivation. Indeed, as previously described for a number of
experimentally analysed genes (e.g. Frías et al., 1994 ; Luque, Flores, &
Herrero, 1994 ; see also Herrero, Muro-Pastor, Valladares, & Flores, 2004 ;
and section 4.1 below), a significant proportion of TSPs in this group are
preceded by sequences matching the canonical (Class II) NtcA-activated
promoter of cyanobacteria. Mitschke et al. (2011) also identified 28 TSPs
whose use was repressed in response to nitrogen limitation. Although these
authors define the NtcA regulon by the TSPs showing transcriptional
changes common to the wild type and the hetR mutant, it should be noted
that a number of hetR -regulated promoters of heterocyst differentiation
genes have been experimentally demonstrated to be regulated by NtcA
in a direct manner (see, e.g. Camargo, Valladares, Flores, & Herrero, 2012 ;
Olmedo-Verd,Valladares, Flores, Herrero, & Muro-Pastor, 2008 ). Thus, the
NtcA regulon includes also promoters that in vivo respond to HetR.
The two studies based on deep sequencing performed by Flaherty et al.
(2011) and Mitschke et al. (2011) have shown the presence of multiple non-
coding RNA transcripts, some of which exhibit nitrogen-dependent regu-
lation and, in some cases, HetR-dependent regulation, as well as multiple
antisense transcripts, suggesting antisense transcription to 39% of all genes
in the genome of Anabaena sp. strain PCC 7120. Antisense transcription and
noncoding RNAs perhaps underlie important mechanisms of regulation
during the response to combined-nitrogen deprivation.
3.5.4. Microarray analysis in Nostoc punctiforme
Two DNA microarray-based transcriptomic analyses of the response to
nitrogen limitation have been performed using Nostoc punctiforme ( Campbell
et al., 2007 ; Christman, Campbell, & Meeks, 2011 ). In addition to producing
heterocysts, the vegetative cells of this cyanobacterium can differentiate into
hormogonial cells and akinetes. Campbell et al. (2007) focused on the com-
parison of steady-state cultures of a spontaneous hormogonium-deficient