Mnt2p and Mnt3p of Saccharomyces cerevisiae are members of the Mnn1p family of alpha-1,3-mannosyltransferases responsible for adding the terminal mannose residues of O-linked oligosaccharides.

The genome of Saccharomyces cerevisiae contains five genes that encode type II transmembrane proteins with significant amino acid similarity to the alpha-1,3-mannosyltransferase Mnn1p. The roles of the three genes most closely related to MNN1 were examined in mutants carrying single and multiple combinations of the disrupted genes. Paper chromatographic analysis of [2-3H]mannose-labeled O-linked oligosaccharides released by beta-elimination showed that the MNT2 (YGL257c) and MNT3 (YIL014w) genes in combination with MNN1 have overlapping roles in the addition of the fourth and fifth alpha-1,3-linked mannose residues to form Man4 and Man5 oligosaccharides whereas MNT4 (YNR059w) does not appear to be required for O-glycan synthesis.     

Studies on Infectious Mononucleosis: II: Attempts at Experimental Transmission to Monkeys

Heparinized blood specimens obtained from two patients in the acute phase of infectious mononucleosis and from a healthy technician were injected intravenously into rhesus monkeys (Macaca mulatta) and the animals were observed for three weeks to one month for clinical, hematological and serological signs of infectious mononucleosis.Splenomegaly was the only definite clinical finding after 12 and 16 days, respectively. There were no definite hematological changes.At autopsy, hyperplasia of the germinal centres of lymphoid follicles, occasional foci of lymphocytic infiltration in the red pulp, and abnormal lymphoid cells in venules or arteries of the spleen were noted.The lesions in the spleen suggest that asymptomatic, presumably viral, infections occur in rhesus monkeys after inoculation with material from patients with infectious mononucleosis.     

Altered phosphorylation of topoisomerase I following overexpression in an ovarian cancer cell line.

There is a growing interest regarding the use of camptothecins (CPTs) for the management of ovarian cancer. Since topoisomerase I has been established as a prime target of these drugs in other experimental models, it was important to determine whether sensitivity to CPTs in ovarian cancer cells is also correlated with the cellular level of this enzyme. Despite the 7-fold increase in topoisomerase expression achieved by adenovirus-mediated expression, the sensitivity to a CPT derivative (topotecan), was not improved compared with control cells harboring an endogenous level of the enzyme. This observation is in accordance with the similar level of topoisomerase I activity found in control and overexpressing cells and suggests that these cells may efficiently regulate the enzyme activity. Indeed, topoisomerase I overexpressing cells are characterized by a lack of alkaline phosphatase sensitivity and elimination of the hyperphosphorylated form of the protein. Taken together, these observations strongly suggest that an alteration in the phosphorylation state of topoisomerase I could limit its activity and prevent improvement of CPT response in ovarian cancer cells. In addition, a limited extent of topoisomerase I phosphorylating activity was found in nuclear extract of OVCAR-3 cells. Hence, providing enhancement in topoisomerase I expression may not result in improvement of CPT response in ovarian cancer cells because of an efficient control of the phosphorylation state of the enzyme.