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genomic sequence of A. variabilis has also been recently made available
( http://genome.jgi-psf.org/anava/anava.home.html ) , and DNA can also
be transferred to this strain ( Zahalak, Pratte, Werth, & Thiel, 2004 ), it has
been used in the genetic analysis of heterocyst differentiation less exten-
sively than Anabaena sp. strain PCC 7120 or Nostoc punctiforme . Anabaena
sp. strain PCC 7120 is also frequently referred to as Nostoc sp. but because
the production of hormogonia in this strain is not obvious and its genomic
sequence is highly similar to that of Anabaena variabilis but much less to that
of Nostoc punctiforme , we will keep denoting it as Anabaena sp. strain PCC
7120. Although Anabaena sp. strain PCC 7120 has been more extensively
used in genetic analysis than Nostoc punctiforme , both organisms have been
subjected to transposon mutagenesis permitting the identification of
many heterocyst differentiation genes ( Cohen et al., 1994 ; Wolk, Cai, &
Panoff, 1991 ) and have been used in global transcriptomic analyses pro-
viding wide information on gene expression during heterocyst differentia-
tion or in the mature heterocyst ( Campbell, Summers, Christman, Martin,
& Meeks, 2007 ; Christman et al., 2011 ; Ehira & Ohmori, 2006a ; Flaherty,
van Nieuwerburgh, Head, & Golden, 2011 ; Mitchske et al., 2011 , see
section 3.5 below).
2. THE HETEROCYST
2.1. The Heterocysts are Sites of Nitrogen Fixation
Heterocyst-forming cyanobacteria have been known for over 200 years
( Rippka et al., 1979 ), but the role of the heterocysts in the biology of these
organisms remained elusive for a long time, to the point that heterocysts
were considered 'a botanical enigma' ( Fritsch, 1951 ). Because heterocyst
production negatively correlates with the availability of combined nitrogen
( Fogg, 1949 ) and there is a relationship between the presence of heterocysts
and the capability to fix atmospheric nitrogen, Fay, Stewart, Walsby, and
Fogg (1968) asked whether the heterocysts are the sites of nitrogen fixation
in the filaments.
Nitrogen fixation is carried out in different bacteria by a strongly con-
served enzyme complex, nitrogenase, which carries out the reduction of N 2
producing two molecules of ammonia in a reaction that requires reductant
and energy in the form of ATP ( Rubio & Ludden, 2008 ). Nitrogenase is
made of the dimeric Fe protein (nitrogenase reductase) and the tetrameric,
α 2 β 2 , Fe-Mo protein (properly, nitrogenase) that bears the unique metal
cofactor FeMoCo ( Rubio & Ludden, 2008 ). The nitrogenase complex is
 
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