新生嘌呤生物合成途径的酶可能形成一个多酶复合物,以促进构成途径的十个连续步骤的底物通量。复合物形成的一个可能策略是使用细胞骨架网络或亚细胞膜等结构支架介导蛋白质-蛋白质相互作用。为确定这种策略是否适用于新生嘌呤酶,对第三个嘌呤酶-甘氨酰核苷酸转移酶(GAR Tfase)在活体大肠杆菌和哺乳动物细胞中的定位模式进行了监测。将编码人类和大肠杆菌GAR Tfase的基因融合到绿色荧光蛋白(GFP)中,并通过调节蛋白质表达来引入它们的各自细胞中,并使用共聚焦荧光显微镜监测定位模式。在两种情况下,图像显示蛋白质分散在整个细胞质中。因此,GAR Tfase没有定位到现有的细胞结构中,因此这种设备可能不用于集中途径成员。然而,途径的离散聚簇可能仍然存在于整个细胞质中。
A gene designated purU has been identified and characterized. purU is adjacent to tyrT at min 27.7 on the Escherichia coli chromosome. The gene codes for a 280-amino-acid protein. The C-terminal segment of PurU from residues 84 to 280 exhibits 27% identity with 5'-phosphoribosylglycinamide (GAR) transformylase, the product of purN. Primer extension mapping and assays of lacZ in a promoter probe vector identified two promoters giving mono- and bi-cistronic purU mRNA. Neither mRNA was regulated by purines. Mutations in either of two pairs of genes are required to block synthesis of 5'-phosphoribosyl-N-formylglycinamide (FGAR) from GAR: purN purT (purT encodes an alternative formate-dependent GAR transformylase) or purN purU. On the basis of the growth of purU, purN, and purU purN mutants, it appears that PurU provides the major source of formate for the purT-dependent synthesis of FGAR.
Metabolic enzymes show a high degree of redundancy, and for that reason they are generally ignored in searches for novel targets for anti-infective substances. The enzymes PurN and PurT are redundant
We demonstrate here that Escherichia coli synthesizes two different glycinamide ribonucleotide (GAR) transformylases, both catalyzing the third step in the purine biosynthetic pathway. One is coded for by the previously described purN gene (GAR transformylase N), and a second, hitherto unknown, enzyme is encoded by the purT gene (GAR transformylase T). Mutants defective in the synthesis of the purN- and the purT-encoded enzymes were isolated. Only strains defective in both genes require an exogenous purine source for growth. Our results suggest that both enzymes may function to ensure normal purine biosynthesis. Determination of GAR transformylase T activity in vitro required formate as the C1 donor. Growth of purN mutants was inhibited by glycine. Under these conditions GAR accumulated. Addition of purine compounds or formate prevented growth inhibition. The regulation of the level of GAR transformylase T is controlled by the PurR protein and hypoxanthine.