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dehydrogenase (
Pernil et al., 2010
) and
invB
encoding a heterocyst-specific
invertase (
López-Igual et al., 2010
).
3.4. Genes Repressed during Differentiation
The expression of many genes has been shown to decrease in
Anabaena
sp.
strain PCC 7120 during a time course after combined nitrogen depriva-
tion. Among these, transcripts of genes that encode phycobiliproteins are
conspicuous. These and the transcripts of genes related to photosystem I,
photosystem II, chlorophyll antenna, photosynthetic electron transport and
ATP synthesis drop in abundance early but return to the initial levels at 24 h
(
Ehira & Ohmori, 2006a
; see section 3.5 below). These decreases detected
at the whole filament level should reflect a phenomenon that takes place
throughout the filament. In contrast, transcripts of genes of the Calvin cycle
and gluconeogenesis decreased to about half of the initial levels by 8 h after
nitrogen deprivation. In the case of
rbcL
(encoding a subunit of Rubisco;
Curatti, Giarrocco, & Salerno, 2006
;
Elhai & Wolk, 1990
) and
susA
(encod-
ing the sucrose cleavage enzyme sucrose synthase;
Curatti et al., 2006
), it
has been shown, by making use of transcriptional fusions, that repression
takes place specifically in (pro)heterocysts. Apart from metabolism genes,
remarkable cases of genes repressed in heterocysts are those of
glnB
encod-
ing the P
II
protein (
Paz-Yepes et al., 2009
), whose repression may be related
to the need of activity of PipX (
Valladares et al., 2011
), for which the P
II
has
been proposed to be an antagonist in the unicellular cyanobacterium
Syn-
echococcus elongatus
(
Llácer et al., 2010
), and
ftsZ
(
Wang & Xu, 2005
), whose
repression may be related to the nondividing, differentiation-at-terminus
character of the heterocyst.
3.5. Global Studies of Gene Expression
Several global transcriptional analyses of the responses of heterocyst-
forming cyanobacteria to combined nitrogen deprivation have been
published recently. Two main types of analysis have been performed, one
based on DNA microarrays and the other based on deep-sequencing
technology. In addition to the technique used to detect the transcription
levels, there are some differences between the different analyses con-
cerning the incubation conditions of the cyanobacteria. The microarray
analysis of
Ehira & Ohmori (2006a)
was performed using filaments
of
Anabaena
sp. strain PCC 7120 and an
nrrA
mutant (see section 4.3
below) grown with nitrate as the nitrogen source and transferred to
a medium lacking combined nitrogen (liquid cultures bubbled with
