Cobalt activation of Escherichia coli 5'-nucleotidase is due to zinc ion displacement at only one of two metal-ion-binding sites.

Escherichia coli 5'-nucleotidase activity is stimulated 30- to 50-fold in vitro by the addition of Co(2+). Seven residues from conserved sequence motifs implicated in the catalytic and metal-ion-binding sites of E. coli 5'-nucleotidase (Asp(41), His(43), Asp(84), His(117), Glu(118), His(217) and His(252)) were selected for modification using site-directed mutagenesis of the cloned ushA gene. On the basis of comparative studies between the resultant mutant proteins and the wild-type enzyme, a model is proposed for E. coli 5'-nucleotidase in which a Co(2+) ion may displace the Zn(2+) ion at only one of two metal-ion-binding sites; the other metal-ion-binding site retains the Zn(2+) ion already present. The studies reported herein suggest that displacement occurs at the metal-ion-binding site consisting of residues Asp(84), Asn(116), His(217) and His(252), leading to the observed increase in 5'-nucleotidase activity

Biased codon usage in signal peptides: A role in protein export

The signal peptide of proteins exported via the general secretory pathway encodes structural features that enable the targeting and export of the protein to the periplasm. Recent studies have shown biased codon usage at the second amino acid position and a high usage of non-optimal codons within the signal peptide. Altering these biases in codon usage can have deleterious effects on protein folding and export. We propose that these codon-usage biases act in concert to optimize the export process through modulating ribosome spacing on the transcript. This highlights a new aspect of protein export and implies that codon usage in the signal peptide encodes signals that are important for protein targeting and export to the periplasm

Hypoxanthine-guanine phosphoribosyltransferase deficiency: analysis of HPRT mutations by direct sequencing and allele-specific amplification

The Lesch-Nyhan syndrome is a severe X chromosome-linked human disease caused by a virtual absence of hypoxanthine-guanine phosphoribosyltransferase (HPRT) activity. A partial deficiency in the activity of this enzyme can result in gouty arthritis. To determine the genetic basis for reduction or loss of enzyme activity, we have amplified and sequenced the coding region of HPRT cDNA from four patients: one with LeschNyhan syndrome (HPRTPerth) and three with partial deficiencies of HPRT activity, which have been designated HPRTUrangan, HPRTSwan and HPRTToowong. In all four patients, the only mutation identified was a single base substitution in exons 2 or 3 of the coding region, which in each case predicts a single amino acid substitution in the translated protein. Each base change was confirmed by allele-specific amplification of the patient's genomic DNA. It is interesting to note that the mutation found for HPRTPerth is identical to that reported for HPRTFlint. It appears that the two mutations are de novo events

Expression of active human hypoxanthine-guanine phosphoribosyltransferase in Escherichia coli and characterisation of the recombinant enzyme

A plasmid, pRG1, has been constructed by incorporating the coding sequence of human hypoxanthine-guanine phosphoribosyltransferase (HPRT) into the expression vector pT7-7. Expression of human HPRT has been achieved in HPRT Escherichia coli cells transformed with pRG1 and pGP1-2, as shown by: (1) exclusive labelling with [S]methionine of a polypeptide with the same mobility as purified human HPRT on SDS-PAGE; and (2) measurement of HPRT activity after cell lysis. Although the majority of the recombinant HPRT was present in the particulate fraction after cell lysis and centrifugation, sufficient HPRT activity was present in the supernatant fraction to allow comparison with the HPRT purified from human erythrocytes and the activity in human haemolysates and lymphoblast lysates. Small differences in electrophoretic mobility on native gels were found between HPRT activity from these sources. The K values of recombinant HPRT for the substrates 5-phospho-α-D-ribosyl-1-pyrophosphate and guanine were compared with those of lymphoblast and erythrocyte HPRT