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ISSN 1672-9145 Acta Biochim Biophys Sin 2005, 37(4): 221–226 CN 31-1940/Q

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nifH Promoter Activity Is Regulated by DNA Supercoiling in Sinorhizobium meliloti

Yan-Jie LIU, Biao HU, Jia-Bi ZHU, Shan-Jiong SHEN, and Guan-Qiao YU*

National Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, China

Abstract In prokaryotes, DNA supercoiling regulates the expression of many genes; for example, the expression of Klebsiella pneumoniae nifLA operon depends on DNA negative supercoiling in anaerobically grown cells, which indicates that DNA supercoiling might play a role in gene regulation of the anaerobic response. Since the expression of the nifH promoter in Sinorhizobium meliloti is not repressed by oxygen, it is proposed that the status of DNA supercoiling may not affect the expression of the nifH promoter. We tested this hypothesis by analyzing nifH promoter activity in wild-type and gyr^– Escherichia coli in the presence and absence of DNA gyrase inhibitors. Our results show that gene expression driven by the S. meliloti nifH promoter requires the presence of active DNA gyrase. Because DNA gyrase increases the number of negative superhelical turns in DNA in the presence of ATP, our data indicate that negative supercoiling is also important for nifH promoter activity. Our study also shows that the DNA supercoiling-dependent S. meliloti nifH promoter activity is related to the trans-acting factors NtrC and NifA that activate it. DNA supercoiling appeared to have a stronger effect on NtrC-activated nifH promoter activity than on NifA-activated promoter activity. Collectively, these results from the S. meliloti nifH promoter model system seem to indicate that, in addition to regulating gene expression during anaerobic signaling, DNA supercoiling may also provide a favorable topology for trans-acting factor binding and promoter activation regardless of oxygen status.

Key words nifH; DNA supercoiling; gyrase; Sinorhizobium meliloti; gene expression

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Received: January 10, 2005 Accepted: February 24, 2005

This work was supported by the grants from the National Natural Science Foundation of China (No. 30170512) and the Major State Basic Research Development Program of China (No. 2001cb108901)

*Corresponding author: Tel, 86-21-54924165; Fax, 86-21-54924015; E-mail, [email protected]