Nitrate reductase activity in isolated heterocysts of the cyanobacterium Nostoc muscorum

Abstract
The nitrate reductase, nitrate reductase apoprotein and nitrate reductase Mo‐cofactor activities were measured in the cell‐free extracts of isolated heterocysts and whole filaments of the cyanobacteriumNostoc muscorum. The nitrate reductase activity of N2‐ and NO 3‐grown whole filaments was 3.25 and 7.14 nmol NO 2 formed·min−1·mg protein−1, respectively. No nitrate reducatase activity was found in the extracts of isolated heterocysts. However, when the heterocyst extract was supplemented with nitrate reductase apoprotein from the whole filaments, a reconstituted nitrate reductase activity of 6.4 nmol NO 2 2 formed·min−1·mg protein−1 was detectable. Therefore, it was concluded that nitrate reductase in Nostoc muscorum was localized in the vegetative cells, that the heterocysts lacked nitrate reductase activity due to the lack of apoprotein, and that the Mo‐cofactor was present in heterocysts.

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