英语翻译
NrrA Directly Regulates Expression of hetR during Heterocyst
Differentiation in the Cyanobacterium Anabaena sp.Strain PCC 7120\1
Heterocysts are terminally differentiated cells of cyanobacteria
that are capable of fixing atmospheric nitrogen.Upon
limitation of combined nitrogen in the medium,particular vegetative
cells change their form into heterocysts with a regular
spacing of 10 to 15 cells (10,12,29).In the filamentous cyanobacterium
Anabaena sp.strain PCC 7120,about 10% of
genes on the chromosome are estimated to be upregulated
during heterocyst development (6,7).Heterocysts markedly
differ from vegetative cells in physiological and morphological
aspects (30).Heterocyst development provides an excellent
model for studying cell differentiation and multicellular pattern
formation.
The hetR gene is a master gene that regulates heterocyst
differentiation.While mutation in the hetR gene blocks early
steps in differentiation,extra copies of hetR result in the formation
of multiple contiguous heterocysts (Mch phenotype)
under nitrogen-depleted conditions (4).Expression of hetR
from the petE promoter,which is regulated by copper,induces
heterocyst differentiation irrespective of cellular nitrogen status
(5).The level of the hetR transcripts increases shortly after
nitrogen deprivation,and its expression is localized to heterocysts
(2).The upregulation of hetR depends on ntcA (9).
The ntcA gene,which is required for heterocyst development
(9,28),encodes a transcriptional regulator that globally controls
nitrogen metabolism in cyanobacteria (19,27).The NtcA
protein,belonging to the cyclic AMP receptor protein family of
bacterial regulators,binds to promoter regions containing the
sequence signature GTAN8TAC (12),and an eight-base palindromic
sequence,TGTAN8TACA,is shown to be the optimal
NtcA-binding sequence by in vitro selection (15).The
DNA-binding activity of NtcA is enhanced by 2-oxoglutarate
(2-OG) (17,26),and NtcA-dependent transcription from the
glnA promoter of Synechococcus sp.strain PCC 7942 increases
in the presence of 2-OG in vitro (25).2-OG is proposed to be
a signaling molecule that transmits information regarding cellular
nitrogen status (24) and that triggers heterocyst differentiation
(17).The NtcA-dependent promoters of hetR do not
contain the NtcA recognition sequence,and NtcA does not
bind to DNA fragments corresponding to the hetR promoter
regions (23).Thus,the molecular basis of the NtcA-dependent
regulation of hetR remains unknown.
Recently we found that the nrrA gene (all4312),encoding a
response regulator of the OmpR family,was involved in the
regulation of heterocyst development (6).The nrrA gene is
upregulated by nitrogen deprivation,but the induction of nrrA
does not take place in the ntcA mutant strain (6,22).