Bb. Singh et al., Identification of amino acids responsible for the oxygen sensitivity of ferredoxins from Anabaena variabilis using site-directed mutagenesis, BBA-BIOENER, 1412(3), 1999, pp. 288-294
The filamentous cyanobacterium Anabaena variabilis (ATCC 29413) possesses t
wo molybdenum dependent nitrogenase systems, nif1 and nif2. The nif1 system
is regulated by a developmental program involving heterocyst differentiati
on; the nif2 system is expressed in all cells only under anaerobic conditio
ns and the expression is controlled environmentally. The genes fdxH1 and fd
xH2, encoding two [2Fe-2S] ferredoxins, are part of the these two distinct
and differently regulated nif gene clusters. The sensitivity of both ferred
oxins to oxygen was different; the half-life of FdxH2 in air was only appro
ximate to 1.5 h, while FdxH1 retained 80% of its nitrogenase activity after
24 h. We used site-directed mutagenesis to identify the role of individual
amino acid residues responsible for oxygen sensitivity and found out that
the FdxH2 double mutant I76A/V77L was much more resistant to oxygen than th
e wild-type ferredoxin (FdxH2) and similar to FdxH1. By modelling it was sh
own that the accessibility of the cavity around the iron-sulfur cluster was
responsible for that. (C) 1999 Elsevier Science B.V. All rights reserved.