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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2996 |
| Date | 15 May 2009 |
| Creators | Aldea, Maria Ramona |
| Contributors | Golden, James |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | electronic, application/pdf, born digital |
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