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As it will be described, in addition to these metabolites, some regula-
tors appear to be transferred between cells in the cyanobacterial filament.
Possible paths of transfer are starting to be understood.
The outer membrane of the cell envelope is continuous along the fila-
ment in heterocyst-forming cyanobacteria (see Flores et al., 2006 ; Wilk et al.,
2011 ) defining a continuous periplasm that can constitute a communication
path between the cells in the filament ( Mariscal, Herrero, & Flores, 2007 ).
In favour of the view that the periplasm is a communication conduit, the
outer membrane has been found to be relatively impermeable to metabo-
lites such as sucrose and glutamate that are important in the diazotrophic
physiology ( Nicolaisen et al., 2009b ). In this scenario, transporters mediating
the transfer of substrates between the periplasm and the cytoplasm would
be important for diazotrophy. Such transporters are known at least for some
amino acids ( Picossi et al., 2005 ). Additionally, at least two types of protein
complexes appear to link directly adjacent cells in the filament, those con-
taining SepJ and those containing FraC/D ( Merino-Puerto et al., 2011 ). All
these proteins have been found to reside in the intercellular septa along the
filament ( Flores et al., 2007 ; Merino-Puerto, Mariscal, Mullineaux, Herrero,
& Flores, 2010 ) and to be required for the intercellular transfer of some flu-
orescent tracers ( Merino-Puerto et al., 2010 , 2011 ; Mullineaux et al., 2008 ).
The structures containing these proteins might correspond to those termed
microplasmodesmata that have been observed by conventional transmission
electron microscopy ( Lang & Fay, 1971 ) and by freeze-fracture electron
microscopy ( Giddings & Staehelin, 1978 ), and that have been recently sug-
gested to be named septosomes after their observation by electron tomog-
raphy ( Wilk et al., 2011 ). Which metabolites and regulators are transferred
through each of these paths (the periplasm, the SepJ-containing and the
FraC/D-containing septal complexes) remain to be established.
3. THE SPECIFIC PROGRAM OF GENE EXPRESSION
3.1. Triggering of the Differentiation Process
As mentioned above, the presence of heterocysts in a cyanobacterial filament
negatively correlates with the availability of combined nitrogen. Consistently,
nitrogen starvation is an environmental cue that determines heterocyst dif-
ferentiation. The impact of nitrogen starvation on metabolism depends, how-
ever, on the availability of other nutrients, notably sources of carbon. Thus,
when carbon is readily available, nitrogen starvation results in a high carbon-
to-nitrogen balance in the cell, whereas for limiting carbon, the impact of
 
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