Molecular cloning and characterization of the purE operon of Escherichia coli

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Purification and characterization of the purE, purK, and purC gene products: identification of a previously unrecognized energy requirement in the purine biosynthetic pathway

Aminoimidazole riobnucleotide carboxylase, the sixth step in the purine biosynthetic pathway, catalyzes the conversion of aminoimidazole ribonucleotide (AIR) to carboxyaminoimidazole ribonucleotide (CAIR). The gene products of the purE and purK genes (PurE and PurK, respectively) thought to be responsible for this activity have been overexpressed and the proteins purified to homogeneity. PurE separates from PurK in the first ammonium sulfate fractionation during the purification. No evidence for association of the two gene products under a variety of conditions using a variety of methods could be obtained. To facilitate the assay for CAIR production, the purC gene product, 5-aminoimidazole-4-N-succinylcarboxamide ribonucleotide (SAICAR) synthetase has also been overexpressed and purified to homogeneity. The activities of PurE, PurK, and PurE.PurK have been investigated. PurE alone is capable of catalyzing the conversion of AIR to CAIR 1 million times faster than the nonenzymatic rate. The Km for HCO3- in the PurE-dependent reaction is 110 mM! PurK possesses an ATPase activity that is dependent on the presence of AIR. No bicarbonate dependence on this reaction could be demonstrated (less than 100 microM), and AIR is not carboxylated during the hydrolysis of ATP. Incubation of a 1:1 mixture of PurE and PurK at low concentrations of bicarbonate (less than 100 microM) revealed that CAIR is produced but requires the stoichiometric conversion of ATP to ADP and Pi. No dependence on the concentration of HCO3- could be demonstrated. A new energy requirement in the purine biosynthetic pathway has been established.

Nucleotide sequence analysis of the purEK operon encoding 5'-phosphoribosyl-5-aminoimidazole carboxylase of Escherichia coli K-12

5'-Phosphoribosyl-5-aminoimidazole (AIR) carboxylase (EC 4.1.1.21) catalyzes step 6, the carboxylation of AIR to 5'-phosphoribosyl-5-aminoimidazole-4-carboxylic acid, in the de novo biosynthesis of purine nucleotides. As deduced from the DNA sequence of restriction fragments encoding AIR carboxylase and supported by maxicell analyses, AIR carboxylase was found to be composed of two nonidentical subunits. In agreement with established complementation data, the catalytic subunit (deduced Mr, 17,782) was encoded by the purE gene, while the CO2-binding subunit (deduced Mr, 39,385) was encoded by the purK gene. These two genes formed an operon in which the termination codon of the purE gene overlapped the initiation codon of the purK gene. The 5' end of the purEK mRNA was determined by mung bean nuclease mapping and was located 41 nucleotides upstream of the proposed initiation codon. The purEK operon is regulated by the purR gene product, and a purR regulatory-protein-binding site related to the sequences found in other pur loci was identified in the purEK operon control region.

Identification and sequence analysis of Escherichia coli purE and purK genes encoding 5'-phosphoribosyl-5-amino-4-imidazole carboxylase for de novo purine biosynthesis

It has been shown that the Escherichia coli purE locus specifying 5'-phosphoribosyl-5-amino-4-imidazole carboxylase in de novo purine nucleotide synthesis is divided into two cistrons. We cloned and determined a 2,449-nucleotide sequence including the purE locus. This sequence contains two overlapped open reading frames, ORF-18 and ORF-39, encoding proteins with molecular weights of 18,000 and 39,000, respectively. The purE mutations of CSH57A and DCSP22 were complemented by plasmids carrying ORF-18, while that of NK6051 was complemented by plasmids carrying ORF-39. Thus, the purE locus consists of two distinct genes, designated purE and purK for ORF-18 and ORF-39, respectively. These genes constitute a single operon. A highly conserved 16-nucleotide sequence, termed the PUR box, was found in the upstream region of purE by comparing the sequences of the purF and purMN operons. We also found three entire and one partial repetitive extragenic palindromic (REP) sequences in the downstream region of purK. Roles of the PUR box and REP sequences are discussed in relation to the genesis of the purEK operon.