The affinities of a range of penicillins and cephalosporins for ther penicillinbinding proteins of Escherichia coli K-12 have been studied, and the results were compared with the antibacterial activity of the compounds against E. coli K-12 and an isogenic permeability mutant. Different penicillins and cephalosporins exhibited different affinities for the "essential" penicillin-binding proteins of E. coli K-12, in a manner which directly correlated with their observed effects upon bacterial morphology. Furthermore, the affinities of the compounds for their "primary" lethal penicillin-binding protein targets showed close agreement with their antibacterial activities against the permeability mutant.

36:733-739, 1992). The effect of 3TC 5'-triphosphate on both the RNA-dependent and DNA-dependent activities of human immunodeficiency virus type 1 reverse transcriptase and DNA polymerases alpha, beta, and gamma from HeLa cells was investigated. 3TC 5'-triphosphate is a competitive inhibitor (with respect to dCTP) of the RNA-dependent DNA polymerase activity (apparent Ki = 10.6 1.0 to 12.4 5.1 ,uM, depending on the template and primer used); the DNA-dependent DNA polymerase activity is 50%o inhibited by a 3TC 5'-triphosphate concentration of 23.4 + 2.5 ,uM when dCTP is present at a concentration equal to its Km value. Chain elongation studies show that 3TC 5'-triphosphate is incorporated into newly synthesized DNA and that transcription is terminated in a manner identical to that found for ddCTP. The 50%o inhibitory concentrations of 3TC 5'-triphosphate against DNA polymerases alpha, beta, and gamma at concentrations of dCTP equal to the Km were 175 31, 24.8 10.9, and 43.8 16.4 ,uM, respectively. More detailed kinetic studies with 3TC 5'-triphosphate and DNA polymerases beta and gamma are consistent with the fact that inhibition of these enzymes by 3TC 5'-triphosphate is competitive with respect to dCTP. The values of Ki were determined to be 18.7 ,uM for DNA polymerase beta and 15.8 0.8 ,uM for DNA polymerase gamma.

The competition of a number of beta-lactam morphogenic probes for the penicillin-binding proteins (PBPs) of Pseudomonas aeruginosa, Enterobacter cloacae, Klebsiella aerogenes, Proteus rettgeri, and Escherichia coli has been studied. The results indicate that the various gram-negative bacteria have similar, but not identical, PBP patterns and that the individual proteins probably perform similar morphogenic functions as in E. coli K-12. Comparison of the 50% binding concentrations of the compounds for the various PBPs of the five strains with their antibacterial activity indicates that the different antibiotics are excluded to a greater or lesser degree by the outer membrane permeability barrier and that the exclusion is most pronounced in P. aeruginosa.

The competition of a new aminothiazolyl cephalosporin, ceftazidime, for the penicillin-binding proteins (PBP's) of Escherichia coli K12, Pseudomonas aeruginosa and Staphylococcus aureus has been studied. Ceftazidime caused filamentation and eventually cell lysis of both E. coli and Ps. aeruginosa at its minimum inhibitory concentration, due to its primary activity against PBP-3. The antibiotic also inhibited PBP's 1 a and 1 bs, the 'essential' cell elongation proteins at higher, therapeutically achievable concentrations and consequently induced rapid lysis of both E. coli and Ps. aeruginosa. In Staph. aureus ceftazidime showed high affinity for PBP-1, -2 and less affinity for PBP-3. The results indicate that in E. coli K12 and Ps. aeruginosa, ceftazidime owes its good antibacterial activity to high affinity for PBP-3, the 'essential' binding protein involved in cell division combined with favourable outer membrane penetration.

Kinetic constants were determined for the hydrolysis of a series of synthetic peptide substrates by recombinant rhinovirus (HRV 14) 3C proteinase. Systematic removal or replacement of individual residues indicated that the minimum sequence required for effective cleavage by the viral cysteine proteinase was P,-Val/Thr-P,-P,-Gln-Gly-Pro.