Gene Expression during Heterocyst Differentiation - Advances in Botanical Research - page 295

CO 2 -enriched air). Samples were taken at time 0 and 3, 8 and 24 h
after nitrate withdrawal. The deep-sequencing analysis carried out by
Flaherty et al. (2011) was performed using filaments of Anabaena sp.
strain PCC 7120 grown in an ammonium-containing medium and
transferred to a medium lacking combined nitrogen (liquid cultures)
for 6, 12 and 21 h. The results from these two analyses differ specially at
the first time points since the microarray analysis started from filaments
grown in the presence of nitrate, which, in regulatory terms, is a less-
stringent nitrogen source compared to ammonium. Indeed, the master
regulator of nitrogen assimilation genes, NtcA, is active under these con-
ditions, and activates the expression of genes involved in the assimilation
of nitrate and nitrite such as those in the nirA operon ( Frías, Flores, &
Herrero, 1994 ).
3.5.1. Microarray analysis in Anabaena sp. strain PCC 7120
The microarray analysis of Ehira & Ohmori (2006a) , as re-evaluated by Xu
et al. (2008) , showed that the overall abundance of mRNA in wild-type
Anabaena decreased after nitrogen stepdown. This can be explained by a
generalized drop in transcription of the genes most highly expressed under
nitrogen-replete conditions, remarkably those encoding the phycobilipro-
teins. Nonetheless, the authors detected 495 genes that increased and 196
that decreased specifically in expression at some point upon combined-
nitrogen deprivation. They found that 51% of the genes whose expression
increase at 3 h also do so at 8 h, and 71% of the genes whose expression
decrease at 3 h also decrease at 8 h, which indicates a considerable overlap-
ping in the transcriptional patterns operating during these stages of differ-
entiation. In contrast, only 2% of the genes induced at 3 h remain so at 24 h
and only 7% of those induced at 8 h remain so at 24 h.
Some genes encoding proteins involved in the assimilation of urea (the
ABC-type uptake transporter UrtABCDE) and scavenging of ammonium
(Amt translocators) are induced early upon nitrate withdrawal. The expres-
sion of genes in the cph1 cluster for cyanophycin metabolism increases sig-
nificantly at 3 h and remains high at 8 h. However, by 24 h, the levels of
expression of genes involved in cyanophycin metabolism, as well as those of
genes involved in urea and ammonium transport, were similar to the initial
levels. In contrast, at 24 h, the nifH , nifB and nifE gene clusters are con-
spicuously induced. Regarding carbon metabolism, by 8 h expression of the
rbcLS genes specific of the Calvin cycle decreased, whereas those specific for
the oxidative pentose phosphate pathway increased. By 24 h, transcription