RNase-sensitive DNA polymerase activity in cell fractions and mutants of Neurospora crassa

RNase-sensitive DNA polymerase activity (RSDP) was tested in different cell fractions of Neurospora crassa cell types and its morphological mutants. This RSDP was found localized in the microsomal pellet fraction and absent in the purified nuclear pellets isolated from different N. crassa cell types: conidia, germinated conidia, and mycelia. This enzyme is capable of synthesizing a DNA product only in the presence of all four deoxyribonucleoside-5' - triphosphates and Mg2+. Removal of RNA from the pellet fraction by RNase strongly inhibited the DNA synthesis. The endogenous synthesis of DNA in the microsomal pellet fraction was associated with the formation of an RNA:DNA hybrid as analyzed by Cs2SO4 equilibrium density gradient centrifugation. The DNA product after alkali hydrolysis hybridizes with the RNA isolated from the same pellet fraction, as analyzed by elution from hydroxylapatite column at 60C. This DNa product did not hybridize with poly (A). A few mutants tested showed this RNase-sensitive DNA polymerase activity.