Transformation in pneumococcus: nuclease resistance of deoxyribonucleic acid in the eclipse complex.

Donor deoxyribonucleic acid strands in the eclipse phase of genetic transformation of pnuemococcus (Streptococcus pneumoniae) are purified as a complex with a cf the deoxyribonucleic acid strand in this complex to digestion by nucleases was shown to be 50- to 1,000-fold less than that of uncomplexed single strands of deoxyribonucleic acid. Deoxyribonuclease I, micrococcal nuclease, Neurospora endonuclease, nuclease P1, and the major endogenous nuclease of cell-free extracts were studied. Sensitivity to nuclease attack was not uniform along the deoxyribonucleic acid strand; sequences of strongly protected bases were separated by more sensitive regions. The minimum size of protected fragments was about 70 bases. A complex of protein with the protected deoxyribonucleic acid segments was obtained after partial digestion. The sizes of these complexes, of the protected deoxyribonucleic acid segments, and of the protein subunit released by complete nuclease digestion, are all approximately identical, as determined by gel exclusion chromatography. Deoxyribonucleic acid strands of eclipse complex were also shown to be particularly well protected from attack by the major pneumococcal endonuclease in cell extracts.