Gene Expression during Heterocyst Differentiation - Advances in Botanical Research - page 293

By making use of transcriptional or translational fusions to reporter
genes, mainly gfp (encoding the green fluorescent protein) but also luxAB ,
the spatial time course of activation along the filament has been studied
for an increasing number of genes involved in heterocyst differentiation or
function. The expression of some genes that are activated early upon com-
bined-nitrogen stepdown is increased throughout the filament, although at
somewhat higher levels in cells showing a spatial distribution reminiscent
of further (pro)heterocysts. This is the case of, e.g. the nrrA gene encoding a
response regulator with a role in heterocyst differentiation ( Ehira & Ohmori,
2006a ; Muro-Pastor, Olmedo-Verd, & Flores, 2006 see section 4.3 below).
Unfortunately, no studies are available, to the best of our knowledge, for the
spatial distribution of the induction of genes involved in the assimilation of
traces of combined-nitrogen nutrients other than amt1 , which is expressed
at higher levels in vegetative cells than in heterocysts ( Merino-Puerto et al.,
2010 ). On the other hand, genes for which a maximum induction takes
place at intermediate stages of differentiation exhibit a more specific local-
ization to (pro)heterocysts (e.g. the ntcA gene ( Olmedo-Verd, Muro-Pastor,
Flores, & Herrero, 2006 )). It should be taken into account that the relative
high stability of the GFP usually precludes visualization by this approach
of the drop in gene expression for transiently expressed genes as it can be
observed detecting mRNA.
3.3. Genes Active in the Mature Heterocyst
Because of the transient induction of many genes expressed at early to
intermediate times during differentiation, the transcriptional pattern pre-
vailing during a significant part of the differentiation process, which likely
takes place under a condition of nitrogen stress, differs from that taking
place in the mature heterocyst. For a number of genes activated late dur-
ing heterocyst differentiation, induction has been shown to exhibit a tight
spatial specificity, expression remaining high after completion of differen-
tiation. This is the case of the pipX gene, encoding a regulatory factor that
supports late gene expression in the differentiating cells, whose spatio-
temporal specificity has been studied ( Valladares et al., 2011 ). This gene is
activated upon nitrogen stepdown with a burst at c. 9-12 h, which is main-
tained until differentiation is completed (c. 24 h). Also, some genes involved
in heterocyst function have been shown to be activated at medium to late
stages of differentiation and to remain highly expressed in the mature het-
erocyst. This is the case of, e.g. the nifHDK operon encoding nitrogenase
( Elhai & Wolk, 1990 ), the ald gene encoding a heterocyst-specific alanine