On the scent of the regulatory mechanisms controlling sucrose metabolism in Nostoc sp. strain PCC 7120

MARCOZZI C; CUMINO A. C.; SALERNO G. L.

Wisconsin, USA

Workshop; Asilomar - Cyanobacterial Molecular Biology Workshop; 2007

     The function and regulation of sucrose metabolism in cyanobacteria is still not fully understood. In the heterocyst-forming cyanobacterium Nostoc sp. strain PCC 7120, it was shown that genes involved in sucrose synthesis and cleavage (spsB and susA, respectively) exhibit an opposite expression regarding the nitrogen source (1, 2). In cyanobacteria, global nitrogen regulation is operated by NtcA, which is involved in heterocyst development and diazotrophic growth. Here we analyzed the expression patterns from spsB and susA in an NtcA insertional mutant (CSE2) after a nitrogen-step down. When compared with the transcript levels in Nostoc sp. PCC 7120, spsB mRNAII, originated from the transcription start site II (tspII), strongly decreased and the amount of susA mRNAIII raised in the mutant strain CSE2. A similar expression pattern was found at the protein level. However, neither susA mRNAI nor susA mRNAII levels increased in the mutant strain. Two different regions upstream from spsB tspII that exhibited sequence similarity with putative NtcA-binding sites, bound purified recombinant NtcA. Also, NtcA interacted with a down-stream region of susA tspIII. These results suggest that in Nostoc sp. PCC 7120 NtcA is likely to activate the expression of spsB, and to be involved in a complex mechanism in the regulation of susA.  Thus, sucrose metabolism may be coordinated at the transcriptional level integrating signals from nitrogen metabolism. Although we demonstrate that NtcA may be regulating spsB and susA expressions, further studies are needed to understand the intricate regulation network that can be modulating their expression.