McGoldrick H, Deery E, Warren M, Heathcote P
Queen Mary, University of London, London E1 4NS, UK.
Biochem Soc Trans. 2002 Aug;30(4):646-8. doi: 10.1042/bst0300646.
In Rhodobacter capsulatus, cobalamin biosynthesis has been shown to occur when the bacteria are grown either aerobically or anaerobically. However, a comparison of the main cobalamin biosynthetic operon found within R. capsulatus would suggest that the encoded proteins belong to the oxygen-dependent pathway for cobalamin biosynthesis, although, significantly, no homologue of the essential mono-oxygenase CobG has yet been detected. Nonetheless, within this main cob operon is found a large open reading frame termed orf663 that is not found in any other cobalamin biosynthetic operon. When overproduced in Escherichia coli, orf663 was found to encode a 90 kDa integral membrane protein. Some of this protein is cleaved within E. coli to give a soluble N-terminal region that can easily be purified and yields a 50 kDa flavoprotein. When expressed in harness with the genes for precorrin-3a synthesis, ORF663 appears to mediate the transformation of precorrin-3a into a new chromophoric compound. Another open reading frame in close proximity to orf663 is termed orf647, and was found to encode a 2Fe-2S ferredoxin-like protein. We suggest that these two proteins may provide an alternative oxygen-independent mechanism for ring contraction within R. capsulatus.
在荚膜红细菌中，已证明当细菌在需氧或厌氧条件下生长时，钴胺素生物合成会发生。然而，对荚膜红细菌中发现的主要钴胺素生物合成操纵子的比较表明，所编码的蛋白质属于钴胺素生物合成的氧依赖途径，尽管重要的是，尚未检测到必需的单加氧酶CobG的同源物。尽管如此，在这个主要的钴胺操纵子中发现了一个称为orf663的大开放阅读框，在任何其他钴胺素生物合成操纵子中都未发现。当在大肠杆菌中过量表达时，发现orf663编码一种90 kDa的整合膜蛋白。该蛋白的一部分在大肠杆菌中被切割，产生一个可溶的N端区域，该区域可以很容易地纯化，并产生一种50 kDa的黄素蛋白。当与前咕啉-3a合成基因一起表达时，ORF663似乎介导前咕啉-3a转化为一种新的发色化合物。与orf663紧邻的另一个开放阅读框称为orf647，发现它编码一种2Fe-2S铁氧还蛋白样蛋白。我们认为这两种蛋白质可能为荚膜红细菌中的环收缩提供一种替代的非氧依赖机制。
