Mapping the active-site tyrosine of vaccinia virus DNA topoisomerase I.

Site-directed mutagenesis of the vaccinia virus gene encoding a type I DNA topoisomerase implicates Tyr-274 as the active-site residue that forms a covalent adduct with DNA during cycles of DNA-strand breakage and reunion. Replacement of Tyr-274 by phenylalanine results in loss of the ability of the enzyme to relax negatively supercoiled DNA as well as to form the covalent DNA-protein intermediate. Substitution of phenylalanine for tyrosine at nine other sites in the protein has no apparent effect on enzyme activity. Amino acid sequence alignment reveals Tyr-274 to be homologous to Tyr-727 and Tyr-771, respectively, of the type I topoisomerases from Saccharomyces cerevisiae and Saccharomyces pombe; Tyr-727 and Tyr-771 have been shown to represent the active-site tyrosines of those enzymes. Sequence comparison of the active-site regions defines a motif Ser-Lys-Xaa-Xaa-Tyr common to the viral and cellular type I topoisomerases, including the human enzyme.     

Primary hyperparathyroidism and monoclonal gammopathy

Coexistent primary hyperparathyroidism and monoclonal gammopathy, although rare, has been reported previously by a number of investigators. We report four patients with such an occurrence who were seen between 1976 and 1988. Another patient with primary hyperparathyroidism also had multiple myeloma and was in remission for 12 years. These patients represent approximately 1% of the 386 patients with primary hyperparathyroidism seen during the same 12-year period. Although several mechanisms have been proposed to explain this concurrence, we believe it is the result of a chance occurrence. A review of the literature, an estimate of the chance occurrence of coincidental monoclonal gammopathy, benign or malignant, in patients with primary hyperparathyroidism, and some practical implications of this interesting coexistence are presented.     

Suppurative corneal ulceration in Bangladesh

Suppurative keratitis is an important preventable cause of blindness, particularly in the developing world. This study analyses 142 cases of suppurative keratitis referred to Chittagong Eye Infirmary, Bangladesh. Some 53.5% of cases were bacterial and 35.9% were fungal. The five most common pathogens were: Pseudomonas sp. 24%, Streptococcus pneumoniae 17%, Aspergillus sp. 13%, Fusarium sp. 7% and Curvularia sp. 6%. Gram stain and culture results were consistent in 62.6% of cases. Previous antibiotic treatment was a significant factor for failure of culture isolation and less so for Gram stain failure. On Gram stain, 55.9% of pseudomonal cases were missed, but only 2% of fungal cases were missed. Over all, Gram stain had a sensitivity of 62% and positive predictive value of 84% for bacterial cases, and 98% and 94% for fungal cases, respectively. Fungal ulcers were typically filamentous, but an antecedent history of trauma was not common. The most frequent injury was due to rice grains, but the inoculum appeared to be introduced during eye washing with contaminated water. Pseudomonal ulcers occurred most frequently in the monsoon season, and Fusarium cases were seen only in the hot, dry season.