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that these cross-reacting proteins are also globins, no further investigation
has been undertaken. A query of the Trichodesmium erythraeum genome with
the N. commune sequence retrieves no significant hit, which leaves the
question unresolved.
4.2. Nostoc punctiforme
The wealth of genomic information available with the advent of large-scale
sequencing has also resulted in the ability to use a global analysis of gene
expression within the whole cell. These gene expression profiles can give
transcriptional information about the regulation of globins in cyanobacteria.
Although this does not necessarily highlight function, it does showwhen the
cell chooses to up-regulate, or down-regulate, the expression of a gene. This
in turn gives valuable information about conditions under which the pro-
tein's activity is useful. Following the completion of the genome of
N. punctiforme strain ATCC 29133 (PCC 73102; Meeks et al., 2001 ), the
genomic sequences were used to construct DNA microarrays. These arrays
can detect more than 6500 specific genes, with two of these genes predicted
to be globins ( Campbell, Summers, Christman, Martin, & Meeks, 2007 ).
Gene NpR0416 codes for a TrHb1-1 globin (Uniprot ID B2J6Y7) analo-
gous to the glbN gene product found in N. commune UTEX 584, and gene
NpR1005 for a TrHb1-2 globin (Uniprot IDB2IV60; Vuletich & Lecomte,
2006 ). Based upon clustering with orthologous expression groups within the
microarray, NpR0416 is classified as part of the adaptive metabolism of the
cell, linked to metabolic protection, while NpR1005 is a core protein
involved in nitrogen metabolism ( Campbell et al., 2007 ).
In these experiments, RNA is isolated from cells under three different
growth conditions and then compared with RNA isolated from cells grown
in replete medium. Under the first condition, ammonia is withheld from the
medium to favour diazotrophic growth and induce nitrogen fixation. The
second growth condition favours akinete (a spore-like cell type transiently
formed during nutrient stress) formation by extended incubation in the dark
( Argueta & Summers, 2005 ). The third condition favours a switch to a third
cell type, termed hormogonia. This type is formed when the vegetative fil-
aments differentiate into small filaments capable of gliding and buoyant
motility ( Campbell & Meeks, 1989 ). The three cell types each alter gene
regulation and give insight into the relative dependence of the cells on
the two globins. The TrHb1-1 globin (NpR0416) shows up-regulation
during nitrogen depletion, while the TrHb1-2 globin (NpR1005) shows
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