P Plateau

Crystal structure at 1.2 Å resolution and active site mapping of Escherichia coli peptidyl-tRNA hydrolase

Peptidyl‐tRNA hydrolase activity from Escherichia coli ensures the recycling of peptidyl‐tRNAs produced through abortion of translation. This activity, which is essential for cell viability, is carried out by a monomeric protein of 193 residues. The structure of crystalline peptidyl‐tRNA hydrolase could be solved at 1.2 Å resolution. It indicates a single α/β globular domain built around a twisted mixed β‐sheet, similar to the central core of an aminopeptidase from Aeromonas proteolytica. This similarity allowed the characterization by site‐directed mutagenesis of several residues of the active site of peptidyl‐tRNA hydrolase. These residues, strictly conserved among the known peptidyl‐tRNA hydrolase sequences, delineate a channel which, in the crystal, is occupied by the C‐end of a neighbouring peptidyl‐tRNA hydrolase molecule. Hence, several main chain atoms of three residues belonging to one peptidyl‐tRNA hydrolase polypeptide establish contacts inside the active site of another peptidyl‐tRNA hydrolase molecule. Such an interaction is assumed to represent the formation of a complex between the enzyme and one product of the catalysed reaction.

Evidence for a new Escherichia coli protein resembling a lysyl-tRNA synthetase.

The amino acid sequence deduced from the nucleotide sequence of an open reading frame adjacent to the frdA gene of Escherichia coli shows 30.5% identity with the C terminus of Escherichia coli lysyl-tRNA synthetases. The three motifs characteristic of aminoacyl-tRNA synthetases of class 2 are recognizable within this sequence.

Escherichia coli leucine-responsive regulatory protein (Lrp) controls lysyl-tRNA synthetase expression

Using random Tn10 insertion mutagenesis, we isolated an Escherichia coli mutant strain affected in the regulation of lysU, the gene encoding the inducible form of lysyl‐tRNA synthetase. The transposon giving rise to the altered expression of lysU was found inserted within lrp. The latter gene codes for the leucine‐responsive regulatory protein (Lrp) which mediates a global response of the bacterium to leucine. An involvement of Lrp in the regulation of lysU was searched for by using a lysU‐lacZ operon fusion. The following conclusions were reached: (i) inactivation of lrp causes an increased activity of the lysU promoter, whatever the growth conditions assayed, (ii) insertion of a wild‐type lrp gene into a multi‐copy plasmid significantly reduces lysU expression, and (iii) sensitivity of the lysU promoter to the presence of leucine in the growth medium is abolished in the lrp context.

CRYSTALLIZATION AND PRELIMINARY X-RAY ANALYSIS OF ESCHERICHIA COLI PEPTIDYL-TRNA HYDROLASE

Peptidyl‐tRNA hydrolase from Escherichia coli, a monomer of 21 kDa, was overexpressed from its cloned gene pth and crystallized by using polyethylene glycol as precipitant. The crystals are orthorhombic and have unit cell parameters a = 47.24 Å, b = 63.59 Å, and c = 62.57 Å. They belong to space group P212121 and diffract to better than 1.2 Å resolution. The structure is being solved by multiple isomorphous replacement.