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genome sequence in A. thaliana, the genomic and pathway organization of purine biosynthesis in plants appears to be different from that occurring in other organisms. Plant organization seems to be somewhat similar to prokaryotes, but greater differences are found with higher eukaryotes such as Drosophila, chickens, and humans.

Prokaryotes, such as Escherichia coli, have single genes coding monofunctional proteins except for the purH gene which encodes the bifunctional protein 5-aminoimidazole-4-carboxyamide ribonucleotide formyl transferase/inosine-5′-monophosphate (IMP) cyclohydrolase (ATIC, steps 9 and 10, in Table 4.1). Eukaryotes contain single genes encoding mono-, bi- and trifunctional proteins (Smith and Atkins 2002; van der Graaff et al. 2004). The de novo purine biosynthesis pathway in plants consist of monofunctional proteins, except for the bifunctional enzymes AIRC (pur6) and ATIC (pur9/10) (see Table 4.1).

Table 4.1 Summary of enzymes involved in the purine biosynthesis de novo in plants.

				Genea)
Step	Enzyme name	EC	Abbreviation	Plant	E. coli	TAIR locus
1	5-Phosphoribosylpyrophosphate amidotransferase	2.4.2.14	PRAT1	pur1	purF	AT2G16570
			PRAT2			AT4G34740
			PRAT3			AT4G38880
2	Glycineamide ribonucleotide synthase	6.3.4.13	GARS	pur2	purD	AT1G09830
3	Glycineamide ribonucleotide formyl transferase	2.1.2.2	GARFT	pur3	purN	AT1G31220
4	Formylglycinamide ribonucleotide amidotransferase	6.3.5.3	FGAMS	pur4	purL	AT1G74260
5	5-Aminoimidazole ribonucleotide synthetase	6.3.3.1	AIRS	pur5	purM	AT3G55010
6	5-Aminoimidazole ribonucleotide carboxylase	4.1.1.21	AIRC	pur6	purEK	AT2G05140 AT2G37690
7	4-(N-Succinocarboxamide)-5-aminoimidazole synthetase	6.3.2.6	SAICARS	pur7	purC	AT3G21110
8	Adenylosuccinate lyase	4.3.2.2	ASL	pur8/12	purB	AT1G36280 AT4G18840
9	5-Aminoimidazole-4-carboxyamide ribonucleotide formyl transferase	2.1.2.3	AICARFT	pur9/10	purH	AT2G35040
10	IMP cyclohydrolase	3.5.4.10	IMPCH	pur9/10	purH	AT2G35040
11	Adenylosuccinate synthetase	6.3.4.4	ASS	pur11	purA	AT3G57610
12	Adenylosuccinate lyase	4.3.2.2	ASL	pur8/12	purB	AT1G36280 AT4G18840
13	IMP dehydrogenase	1.1.1.205	IMPDH	gua13	guaB	AT1G79470 AT1G16350
14	GMP synthetase	6.3.5.2	GMPS	gua14	guaA	AT1G63660

      a) Gene designations used for plants and E. coli are shown. The plant gene names are according to Senecoff et al. (1996). The gene names used for E. coli are also often used in plant literature.

      The

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