COMPARISON OF THE ENZYMATIC-PROPERTIES OF THE 2 ESCHERICHIA-COLI LYSYL-TRANSFER-RNA SYNTHETASE SPECIES

In Escherichia coli, lysyl-tRNA synthetase activity is encoded by eith er a constitutive lysS gene or an inducible one, lysU, The two corresp onding enzymes could be purified at homogeneity from a Delta lysU and a Delta lysS strain, respectively, Comparison of the pure enzymes, Lys S and LysU, indicates that, in the presence of saturating substrates, LysS is about twice more active than LysU in the ATP-PPi exchange as w ell as in the tRNA(Lys) aminoacylation reaction. Moreover, the dissoci ation constant of the LysU-lysine complex is 8-fold smaller than that of the LysS-lysine complex, In agreement with this difference, the act ivity of LysU is less sensitive than that of LysS to the addition of c adaverine, a decarboxylation product of lysine and a competitive inhib itor of lysine binding to its synthetase, This observation points tla a possible useful role of LysU, under physiological conditions causing cadaverine accumulation in the bacterium. Remarkably, these condition s also induce lysU expression. Homogeneous LysU and LysS were also com pared in Ap,A synthesis. LysU is only 8-fold more active than LysS in the production of this dinucleotide, This makes unlikely that the heat -inducible LysU species could be preferentially involved in the accumu lation of Ap(4)A inside stressed Escherichia coli cells, This conclusi on could be strengthened by determining the concentrations of Ap(4)N ( N = A, C, G, or U) in a Delta lysU as well as in a lysU(+) strain, bef ore and after a 1-h temperature shift at 48 degrees C. The measured co ncentration values were the same in both strains.