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Description:
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The regulatory mechanisms that govern heterocyst development in Anabaena sp . strain
PCC 7120 have been continuously refined over the last two decades . In this work , we
show that three of the sigma factor genes present in the Anabaena sp . strain PCC 7120
genome are developmentally regulated . Time -lapse microscopy of gfp reporter strains
indicated that expression of sigC , sigG , and sigE is upregulated specifically in
differentiating cells at 4 h , 9 h and 16 h , respectively , after induction of heterocyst
development . We proposed that the sigma factors encoded by these genes are involved
in regulation of heterocyst -specific genes whose expression is relatively coincident with
that of sigC , sigG , or sigE . Indeed , inactivation of the sigC gene caused delayed and
reduced expression of genes required for the early stages of heterocyst development , and
caused delayed development . Inactivation of the sigE gene caused a considerable drop in
expression of nifH , a late gene required for nitrogen fixation .
We also provide evidence that c -di -GMP , a novel bacterial second messenger , is
involved in regulating heterocyst development . The all2874 gene encodes a bona fide
diguanylate cyclase , which synthesizes c -di -GMP , and the gene's inactivation resulted in a decreased tendency to form heterocysts ; this phenotype was exacerbated by high light
intensity . We hypothesize that the putative operon all2875 -all2874 senses and relays
information about light conditions and this information is integrated into the decision to
form heterocysts .
Finally , we identified the all0187 gene , which is expressed at 9 h , a time when cells that
have initiated differentiation commit to complete the process . In nitrogen -free medium ,
all0187 mutant filaments formed abnormally long heterocysts and were unable to grow
diazotrophically . Septum formation between heterocysts and their flanking vegetative
cells was incomplete , leaving one or both poles of the heterocysts more opened and
potentially more permeable to oxygen . Despite having nitrogenase activity , the all0187
mutant was unable to grow diazotrophically . We hypothesize that the diazotrophic
growth defect is caused by the inability of the heterocysts to transport fixed nitrogen to
the neighboring vegetative cells . |