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123 genes downregulated at 24 h and steady-state cultures, respectively).The
majority of the 231 genes that were upregulated under both physiological
conditions encode known heterocyst regulatory and function proteins. The
set of 328 genes upregulated at 24 h but not differentially expressed in
steady-state cultures include genes involved in transcriptional regulation,
cofactors, protein polymerization, transport and secondary metabolism,
which would not be necessary once the cells have acclimated to the new
growth condition.
In an attempt to compare some of the results of these global studies, we
have summarized the transcriptional behaviour of some genes involved in
heterocyst differentiation in both Anabaena sp. strain PCC 7120 and Nostoc
punctiforme . Table 8.1 includes genes for which a HetR-dependent TSP (8 h)
has been identified in strain PCC 7120 ( Mitschke et al., 2011 ). Together,
the global studies described above suggest that 500-1000 genes change in
expression during the period from the onset of combined nitrogen with-
drawal to heterocyst performance. More genes (and regulated TSPs) increase
than decrease in expression during acclimation to combined-nitrogen
deprivation. Most early-induced genes are expressed transiently, whereas
many medium- to late-induced genes remain active once heterocyst dif-
ferentiation has been completed, although a fraction of them return to basal
levels of expression in steady-state diazotrophic growth.
4. MECHANISMS OF GENE REGULATION DURING
HETEROCYST DIFFERENTIATION
4.1. The NtcA and HetR Regulators
Gene regulation during heterocyst differentiation is orchestrated by the
combined action of two principal regulators, the global transcriptional reg-
ulator NtcA that activates, and in some cases represses, genes as a function of
the carbon-to-nitrogen balance of the cells, and the differentiation-specific
factor HetR. Mutants lacking either of these regulators do not show any
sign of heterocyst differentiation upon nitrogen deprivation ( Frías et al.,
1994 ; Ramasubramanian, Wei, & Golden, 1994 ). In addition, when studied
at the whole filament level by northern or primer extension analyses, the
increase in gene expression that in the wild-type strain is observed upon
nitrogen stepdown for genes involved in heterocyst differentiation does not
take place in ntcA mutants. Mutation of hetR also impairs the expression of
many genes activated during heterocyst differentiation, although for some
of them the effect is small.
 
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