Site-directed mutagenesis of the Klebsiella pneumoniae nitrogenase. Effects of modifying conserved cysteine residues in the α- and β-subunits

Publisher Website
Google Scholar
Abstract
The five conserved cysteine residues present in the .alpha.-subunit and the three conserved cysteine residues present in the .beta.-subunit of nitrogenase component 1 were individually changed to alanine. Mutations in the .alpha.-subunit at positions 63, 89, 155 and 275 and in the .beta.-subunit at positions 69, 94 and 152 all resulted in a loss of diazotrophic growth and component 1 activity and loss of the normal e.p.r. signal of the component 1 protein. Component 2 activity was retained. Replacement of cysteine-184 in the .alpha.-subunit with alanine greatly diminished, but did not eliminate, diazotrophic growth and component 1 activity. Substitution of serine for cysteine at position 152 in the .beta.-subunit, in contrast with the substitution of alanine at this position, resulted in the formation of active component 1. Replacement of the non-conserved cysteine-112 in the .beta.-subunit with alanine did not greatly perturb diazotrophic growth or the activity of component 1. Extracts prepared from a mutant, with cysteine-275 of the .alpha.-subunit replaced by alanine, complemented extracts of a mutant unable to synthesize the iron-molybdenum cofactor of nitrogenase, indicating that the alanine-275 substitution increases the availability of cofactor. Furthermore extracts of this mutant exhibited an e.p.r. signal similar to that of extracted iron-molybdenum cofactor. These data suggest a role for cysteine-275 as a ligand to the cofactor.

This publication has 18 references indexed in Scilit: