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This Week’s Citation Classic
DEcE’MBER 19, 1983
TMIfHn B I & Lea P 1. The pathway of nitrogen assimilation in plants.
Phytochemistry 15:873-85, 1976.
(Biochemistry Dept.. Rothamsted Experimental Station, Harpenden, Hertfordshire, England]
mate continuously recycles, while ammonia
The review article describes the isolation of
both ferredoxin and NADPH dependent glu- is incorporated into the 2-amino position of
tamate synthase from a range of plant tis- an amino acid.
“In 1973, Miflin became the head of the
sues. The role of glutamate dehydrogenase
(GDH) in ammonia assimilation
is ques- biochemistry department at Rothamsted
15
tioned and additional
N-labelling and in- after spending a sabbatical with Harry
hibitor studies are used as evidence for the
Beevers in Santa Cruz, California. P.J. Lea
joined the department after spending three
operation of the glutamate synthase cycle.
years working with Sir Leslie Fowden at Uni[The SCI® indicates that this paper has been
cited in over 220 publications since 1976, versity College, London, on the aminoacylamaking it the 2nd most-cited paper pub- tion of t-RNA, but had done his PhD research on plant GDH. Both of us were disillished in this journal.]
lusioned with the low levels of chloroplast
CDII, as we knew that intact chloroplasts
could readily convert nitrite to amino acids
in a light dependent reaction and contain
3
Benjamin j. Miftin
high levels of glutamine synthetase. After
and
many
long
hours
of
discussion
we
concluded
Peter j. Lea
that chioroplasts must be able to carry out
Biochemistry Department
the glutamate synthase reaction. A major
Rothamated Experimental Station
stumbling block was separating glutamine
Harpenden, Flertfordshire AL5 2JQ
from glutamate, and we eventually fell back
England
on the tried and tested (but extremely
unpleasant) method of phenollN 113 paper
chromatography used so successfully by
October 31, 1983
Fowden for the separation of nonprotein
1 amino acids. Using isolated intact pea
“In the early 1960s, Sims and Folkes
chloroplast and extracts derived from them
clearly showed the role of the enzyme glu- we were able to show that they could contamate dehydrogenase
(GDH) in the assimi- vert glutamine and 2-oxoglutarate to two
5
lation of [‘ NH ] in the food yeast Candida
molecules of glutamate. The reaction was
3
utilis. Such was the elegance of their work
catalyzed bya glutamate synthase enzyme
that other people broadened their conclu- active with reduced ferredoxin (similar to
sions to include the whole of the2 plant nitrite reductase) rather than reduced pyrikingdom. Although in 1969, Brown, work- dine nucleotides.
ing at Newcastle University in an adjacent
“The review article considered as a Citadepartment to H.J. Miflin, did suggest a tion Classic
marks our first attempt to
novel pathway of ammonia assimilation in draw together all the available evidence to
bacteria, it was thought at the time to only confirm the role of the glutamate synthase
operate under conditions of severe nitrogen cycle in plants. Once the existence of the
limitations. The primary features of this pathway was realised, a lot of previously
pathway, which has become known as the published work slipped neatly into place
glutamate synthase cycle, are the assimila- allowing us to present a reasonably comtion of ammonia into the amide position of plete and coherent picture of ammonia asglutamine, catalyzed by glutamine synihe- similation which we argue took place
tase [E.C. 6.3.1.2] followed by the reductive through the glutamate synthase cycle rather
transfer of this group to the C-2 of 2-oxogluthan via GDH. This view has been confirmed
tarate to give the amino acid glutamate. This many times by subsequent work; the role of
second step is catalyzed by the enzyme glu- the glutamate synthase cycle in ammonia as4
tamate synthase (or L-glutamate:NADP~[or
similation in nitrogen 5fixing root nodules
and in photorespiration in C leaves is still
ferredoxin) oxidoreductase, E.C. 1.4.1.13 [or
E.C. 1.4.7.1]). Thus, one molecule of gluta- an important area of study.” 3
15
1. Sims A P & Folkes B F. A kinetic study of the assimilation of 1 NJ-ammonia and the synthesis of amino acids in
an exponentially growing culture of Candida u/i/is. Proc. Roy. Soc. London Se,. 8 159:479-502, 1964.
(Cited 75 times.)
2. Teespeal I) W, Meer, I L & Brown C M. Synthesis of glutamate in Aerobacter aerogenes by a hitherto unknown
route. Biochemkaf 1. 117:405-7. 1970. (Cited 150 timCs,)
3. ONeal D & Joy K W. Localisation of glutamine synthetase in chloroplasts. Nature 246:61-2. 1973.
4. Calllmore 3 V. Late M, Lea P 3 a Millie B J Purification and properties of two forest of glulamine synthetase
Irons the plant fraction of Phases/us root nodules. P/an/a 157:245-53, 1983.
5. Wallagrore R M, Key, A I, Lea P 1 & Millie B 3. Photosynthesis, photorespiration and nitrogen
metabolism. Plant Cell Env,ron. 6:301-9, 1983.
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