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(Received for publication, December 20, 1995, and in revised form, September 3, 1996)
From the In maize, a small multigene family encodes the
cytosolic isoforms of glutamine synthetase (GS), and five cDNAs,
designated pGS1a, pGS1b, pGS1c, pGS1d, and pGS1e, have been cloned
(Sakakibara, H., Kawabata, S., Takahashi, H., Hase, T., and Sugiyama,
T. (1992) Plant Cell Physiol. 33, 49-58; Li, M., Villemur,
R., Hussey, P. J., Silflow, C. D., Gantt, J. S., and Snustad, D. P. (1993) Plant Mol. Biol. 23, 401-407). This report
describes the identification and enzymatic characterization of the
cytosolic isoforms of GS in maize roots, namely GS1 and GSr. The
purified isoforms, as well as recombinant enzymes that had been
overexpressed in Escherichia coli, were analyzed by
capillary liquid chromatography/electrospray ionization-mass
spectrometry, and GS1 and GSr were identified as the products of the
GS1a/GS1b and GS1c/GS1d genes, respectively. Upon the addition of ammonia to the culture medium, significant amounts
of GSr accumulated and a preferential increase in GS synthetase activity, as compared to GS transferase activity, was found in the root
extract. Assays with the purified recombinant enzymes confirmed that
the specific biosynthetic and synthetase activities of GSr were
1.6-fold higher than those of GS1. Marked differences in stability were
also found between the two isoforms: GSr was more sensitive to heat
than GS1 and octameric aggregates of the subunits of GSr were easily
dissociated to monomers than those of GS1 at low concentrations of
Mn2+ and Mg2+ ions. These characteristics of
the ammonia-induced isoform of GS seem to be physiologically important
for the primary assimilation of external ammonia by roots.
Volume 271, Number 47,
Issue of November 22, 1996
pp. 29561-29568
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
,
,
Department of Applied Biological Sciences,
School of Agricultural Sciences, Nagoya University, Nagoya 464-01, Japan and the ¶ Institute for Protein Research, Osaka University,
Suita, Osaka 565, Japan
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