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for NtcA, which has 2-oxoglutarate as an effector, HetR could be subjected to
post-translational regulation, a possibility that will merit futher research. Other
co-activators of NtcA-dependent transcription appear to exist, as is the case of
PipX ( Valladares et al., 2011 ), asking whether a very elaborated transcriptional
complex, perhaps including alternative RNAP sigma factors ( Aldea et al.,
2007 ), might be acting in transcription of heterocyst-related genes.
Consistent with a specific function of their protein products during the
differentiation process, a number of heterocyst differentiation genes are only
transiently expressed after combined-nitrogen deprivation. However, the
mechanism of downregulation of such genes is unknown. In this context,
the possible role of N 2 -fixation products once heterocyst differentiation is
completed will merit further investigation. Related to this question is that
of regulation during established diazotrophic growth. The cells in the fila-
ment grow and divide, and when the heterocysts are separated by too many
vegetative cells, a cell in between differentiates into a new heterocyst. The
basis for this differentiation process might be different from that of the syn-
chronous heterocyst differentiation that takes place in filaments subjected
to nitrogen stepdown, but possible mechanisms of regulation have been less
extensively investigated. Availability of nitrogen, which might be scarce for
vegetative cells away from a heterocyst, might again have a role ( Wolk &
Quine, 1975 ; Yoon & Golden, 2001 ). Thus, the study of gradients of metab-
olites that might influence gene expression will also be of much interest.
ACKNOWLEDGEMENTS
The authors thank José E. Frías for the micrograph in Fig. 8.1A and Victoria Merino-
Puerto and Iris Maldener for the micrograph in Fig. 8. 1 B. Work in the authors' laboratory is
currently supported by grants CVI-3838 and CVI-6665 (Proyectos de Excelencia, Junta de
Andalucía) and BFU2010-17980 and BFU2011-22762 (Dirección General de Investigación
y Gestión del Plan Nacional de I+D+i), co-financed by FEDER.
REFERENCES
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are upregulated in heterocysts of the cyanobacterium Anabaena sp. strain PCC 7120.
Journal of Bacteriology , 189 , 8392-8396.
Almon, H., & Böhme, H. (1980). Components and activity of the photosynthetic electron
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Biochimica et Biophysica Acta , 592 , 113-120.
Almon, H., & Böhme, H. (1982). Photophosphorylation in isolated heterocysts from the
blue-green alga Nostoc muscorum . Biochimica et Biophysica Acta , 679 , 279-286.
Awai, K., & Wolk, C. P. (2007). Identification of the glycosyl transferase required for synthesis
of the principal glycolipid characteristic of heterocysts of Anabaena sp. strain PCC 7120.
FEMS Microbiology Letters , 266 , 98-102.
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