Biology Reference
In-Depth Information
deprivation, exhibiting both temporal and spatial components of specific-
ity. When the organism senses nitrogen deficiency, likely as an increase in
the carbon-to-nitrogen balance in the cells, an early response consists in
the activation of genes for the scavenging of traces of combined nitrogen
in the form of ammonium, nitrate, nitrite or urea (
Luque & Forchhammer,
2008
; see section 3.5 below). If nitrogen deficiency persists, heterocyst dif-
ferentiation starts and proceeds involving the sequential action of the prod-
ucts of a number of regulatory and structural genes. Many genes involved in
heterocyst differentiation have been identified after the isolation of mutants
unable to grow fixing N
2
under oxic conditions and the study of their phe-
notypes, and recently by global studies.
Genes responsible for heterocyst differentiation are activated in a
sequential manner. Traditionally, three rough categories of genes have been
distinguished according to the time of induction during the differentiation
process: early genes, activated at c. 3-4 h after combined nitrogen step-
down, which include the
hetR
regulatory gene; intermediate genes includ-
ing those that are activated at c. 6-8 h, such as
ntcA
and those involved in
the synthesis and deposition of the Hep layer of the heterocyst envelope;
and late genes, those activated at 12-18 h, including many structural genes
of the heterocyst-specific metabolic features such as those in the
cox2
,
cox3
,
and
nif
operons. It should be noted that the induction of the early het-
erocyst differentiation genes overlaps that of genes involved in a general
response to nitrogen stress including those for the scavenging of traces of
combined nitrogen. Although these groups of genes can be distinguished
in general terms, the specific time points that define them are necessarily
coarse because the classification is based on information obtained in
different laboratories generally using different growth settings (and thus
nonidentical differentiation rates) for a given organism. In spite of this,
valuable information on the sequence of gene activation during heterocyst
differentiation has accumulated from time course northern or RT-PCR
analysis of the expression of individual genes and by global studies. These
studies have shown that many of the genes induced during the course of
heterocyst differentiation are activated transiently, its expression returning
to basal levels at a certain point thereafter. This is especially true for the
genes induced at early or intermediate stages of the process, whose prod-
ucts may be required at a specific moment during differentiation. On the
other hand, the study of the effects of the mutation of particular genes on
the expression of others has provided valuable information on epistatic
relationships.
