Biology Reference
In-Depth Information
Abstract
Some cyanobacteria (oxygenic phototrophs) grow as chains of vegetative cells (fila-
ments or trichomes).When placed inmedia lacking combined nitrogen, some cells
in the filaments differentiate into N
2
-fixing heterocysts.The nitrogen fixation system
is inactivated by oxygen, and the heterocyst provides amicro-oxic environment for
nitrogenase to function. In this review, we first describe the special envelope and
metabolismthatmakestheheterocystmicro-oxic,toconcentratethenontheregula-
tion of gene expression during the process of differentiation. Differentiation starts
as a response to a persistent high cellular carbon-to-nitrogen balance signalled by
2-oxoglutarate,whichresultsinactivationoftheglobaltranscriptionalregulatorNtcA
followedby increasedexpression,mainly localized todifferentiatingcells,of the
hetR
(encoding the differentiation-specific transcription factor HetR) and
ntcA
genes.The
expressionofgenesencodingproteinsthattransformthevegetativecellintoahetero-
cystisthenactivatedwithaspatiotemporalspecificitytoproduceamaturefunctional
heterocyst.Recentglobalanalyseshaveaddedinformationontimecourseandlevels
of gene expression during the process of differentiation, and much information is
also available on thepromoters of a number of thesegenes.Contiguouspromoters
building complex promoter regions are common among heterocyst-related genes.
Understandingthemolecularmechanismofoperationofthesepromoters, including
therolesofHetRandNtcA,isamajorgoalofresearchinthisfield.
1. INTRODUCTION
Filamentous cyanobacteria are organisms that grow as filaments or
trichomes formed by strings of contiguous cells. The trichome is the organ-
ismic unit in these bacteria and mainly consists of vegetative cells that per-
form oxygenic photosynthesis (
Rippka, Deruelles, Waterbury, Herdman, &
Stanier, 1979
). In filamentous cyanobacteria such as those of the genera
Anabaena
and
Nostoc
, some cells in the filament can differentiate into special-
ized cells with different functions (
Flores & Herrero, 2010
). Heterocysts are
differentiated cells specialized in the fixation of atmospheric nitrogen (N
2
)
that do not perform oxygenic photosynthesis. During the differentiation
process, a specific program of gene expression is activated to produce pro-
teins that convert the vegetative cell into a heterocyst, where specific genes
that encode the nitrogen fixation machinery and the needed accompanying
metabolic enzymes are expressed (
Xu, Elhai, & Wolk, 2008
). In the devel-
oped filament bearing the two cell types, vegetative cells and heterocysts
exchange metabolites, with heterocysts providing the vegetative cells with
fixed nitrogen and vegetative cells providing the heterocysts with photosyn-
thate (
Wolk, Ernst, & Elhai, 1994
). Thus, the diazotrophic trichome is a truly
multicellular organism that requires the activity of two interdependent cell
