In studies of competence-deficient mutants of Haemophilus influenzae which absorb deoxyribonucleic acid (DNA) but fail to produce transformants, it was observed that in some mutants the residual transforming activity for different markers varied widely, i.e., produced a ratio effect. One of these mutants, com−56, was studied intensively to determine the cause of the residual efficiency of transformation and the reason for the ratio effect. The residual frequency of transformation was higher for markers considered single-site mutations (like naladixic acid resistance), whereas the least efficient markers tested were those conferring resistance to high levels of streptomycin or novobiocin which are more complex than single-site mutations. Measurement of frequencies of cotransformation indicated that overall genetic linkage was reduced. Transfection was fairly efficient with phage S2 DNA, but not prophage DNA. Donor marker activity could be detected in transformed cell lysates, but not linked to recipient markers in recombinant molecules. Sucrose gradient analysis of such lysates revealed that donor material was associated with recipient DNA in at least normal quantities, but lacked detectable genetic activity. Material from donor DNA labeled with heavy isotopes was incorporated into recipient chromosomal fragments having a density indistinguishable from normal density, unlike the hybrid density recombinant material found in normal cells. No excessive solubilization or nicking of unincorporated donor was detected. It is postulated that this strain contains a hyperactive nuclease, which reduces the effective size of the input DNA during the integration process
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