Site-directed mutagenesis of Cys-92 from the alpha-polypeptide of Phaseolus vulgaris glutamine synthetase reveals that this highly conserved residue is not essential for enzyme activity but it is involved in thermal stability.

The residue Cys-92 from the alpha-polypeptide of Phaseolus vulgaris glutamine synthetase is a highly conserved residue in prokaryotic and eukaryotic glutamine synthetase genes. This cysteine residue was previously proposed as a good candidate for being essential for enzyme activity. We have examined through heterologous expression in Escherichia coli and site-directed mutagenesis the functional importance of this residue.

Functional importance of Asp56 from the alpha-polypeptide of Phaseolus vulgaris glutamine synthetase. An essential residue for transferase but not for biosynthetic enzyme activity.

Replacement of Asp56 by site-directed mutagenesis of the alpha-gene from Phaseolus vulgaris glutamine synthetase heterologously expressed in Escherichia coli produces a complete loss of transferase enzyme activity, thus revealing essentiality of the residue for this particular enzyme activity. This happens independent of Asp56 being replaced by Ala or Glu, suggesting that the essentiality of this residue cannot be attributed to its negative electrical charge. However, a high level of glutamine synthetase biosynthetic specific activity (referred to glutamine synthetase protein, as determined immunologically), is present in D56A and D56E mutants, suggesting that Asp56 is an example of a residue that has a different role in the catalytic mechanism of both enzyme activities of this protein.

Site-directed mutagenesis of Glu-297 from the alpha-polypeptide of Phaseolus vulgaris glutamine synthetase alters kinetic and structural properties and confers resistance to L-methionine sulfoximine.

In this paper we examine the functionality of Glu-297 from the alpha-polypeptide of Phaseolus vulgaris glutamine synthetase (EC 6.3.1.

Regulation of the expression of ferredoxin-nitrite reductase in synchronous cultures of Chlamydomonas reinhardtii.

The regulation of ferredoxin-nitrite reductase--the second enzyme involved in the nitrate assimilatory pathway--in synchronous cultures of C. reinhardtii has been studied both at the activity and protein levels using specific antibodies. During a cycle of 12 h light/12 h dark (12L:12D), ferredoxin-nitrite reductase activity shows a 24-h fluctuation with a maximum in the middle of the light period.