Biology Reference
In-Depth Information
mutant growing with ammonium as the nitrogen source or without com-
bined nitrogen. They identified 495 genes that were differentially expressed
in the N
2
-grown cells, most of which (373) were upregulated.
Christman
et al. (2011)
, on the other hand, described a transcriptional analysis of the
changes produced after nitrogen stepdown in filaments undertaking the
process of heterocyst differentiation. They used the spontaneous hormogo-
nium-deficient mutant grown with ammonium as the nitrogen source and
incubated in a nitrogen-depleted medium for 0.5, 1, 3, 6, 12, 18, and 24 h
(shaken liquid cultures). A total of 1036 genes were significantly up- or down-
regulated along the 24-h induction. The authors presented two classifica-
tions of the regulated genes: the first one grouped the genes by functionality,
with 18% of the genes belonging to adaptive metabolism and 32% to core
metabolism, and the second grouped the genes according to their temporal
pattern of expression, defining six clusters of genes. Cluster 4 includes genes
that are highly activated at late time points, such as the
hgl
and
nif
genes.
Cluster 6 includes the genes with the highest expression at 12 h upon
induction, such as the
hep
genes. Overall, the timeframes at which these
genes are induced are the same as in
Anabaena
sp. strain PCC 7120 (
Ehira &
Ohmori, 2006a
;
Flaherty et al., 2011
;
Xu et al., 2008
).
Campbell, Christman, and Meeks (2008)
have also compared the results
of the transcriptomic analysis during heterocyst differentiation in the hor-
mogonium-deficient mutant described above with the results of a similar
transcriptomic analysis carried out in wild-type filaments during hormo-
gonia differentiation in response to nitrogen deprivation. They conclude
that the hormogonium-differentiation program is much more complex
than the heterocyst differentiation program, although there are some com-
mon differentially expressed genes. The expression of
nrrA
, for example, was
activated at 0.5 h of nitrogen deprivation in both processes and remained
elevated throughout development, although the induction was more dis-
crete during hormogonia differentiation. However, other regulatory genes
induced during heterocyst differentiation, such as
hetR
or
ntcA
, were not
induced during hormogonia differentiation, which suggests a general role
for
nrrA
in the response to nitrogen stress rather than a specific role in het-
erocyst differentiation (see section 4.3 below).
Christman et al. (2011)
also compared the genes differentially expressed
at 24 h after nitrogen stepdown to the genes expressed differentially in
diazotrophic steady-state cultures (
Campbell et al., 2007
). They found more
genes differentially expressed at 24 h than in steady-stated cultures, most
of which were upregulated (559 vs. 378 genes upregulated and 378 vs.
