PNEUMOCYSTIS-CARINII CONTAINS A FUNCTIONAL CELL-DIVISION-CYCLE CDC2 HOMOLOG

Pneumocystis carinii causes life-threatening pneumonia in immunocompro mised patients. The inability to culture P. carinii has hampered basic investigations of the organism's life cycle, limiting the development of new therapies directed against it. Recent investigations indicate that P. carinii is a fungus phylogenetically related to other ascomyce tes such as Schizosaccharomyces pombe. The cell cycles of S. pombe and homologous fungi are carefully regulated by cell-division-cycle molec ules (cdc), particularly cell-division-cycle 2 (Cdc2), a serine-threon ine kinase with essential activity at the G(1) restriction point and f or entry into mitosis. Antibodies to the otein-serine-threonine-alanin e-isoleucine-arginine (PSTAIR) aminoacid sequence conserved in Cdc2 pr oteins specifically precipitated, from P. carinii extracts, a molecule with kinase activity consistent with a Cdc2-like protein. Cdc2 molecu les exhibit differential activity throughout the life cycle of the org anisms in which they occur. In accord with this, the P. carinii Cdc2 s howed greater specific activity in P. carinii trophic forms (trophozoi tes) than in spore-case forms (cysts). In addition, complete genomic a nd complementary DNA (cDNA) sequences of P. carinii Cdc2 were cloned a nd found to be most closely homologus to the corresponding sequences o f other pathogenic fungi. The function of P. carinii cdc2 cDNA was fur ther documented through its ability to complement the DNA of mutant st rains of S. pombe with temperature-sensitive deficiencies in Cdc2 acti vity. The P. carinii cdc2 cDNA restored normal Cdc2 function in these mutant strains of S. pombe, and promoted fungal proliferation. These s tudies represent the first molecular analysis of the cell-cycle-regula tory machinery in P. carinii. Further understanding of P. carinii's li fe cycle promises novel insights for preventing and treating the intra ctable infection it causes in immunocompromised patients.