In vitro mutation of Haemophilus influenzae transforming deoxyribonucleic acid by ultraviolet radiation at -70 degrees C.

Previous studies have shown the non-mutability of Haemophilus influenzae either by UV irradiation of the cells or by irradiating the transforming DNA and transformation of competent cells. In the present work, we present evidence of transforming DNA mutation in vitro by UV irradiation at -70 degrees C, which upon transformation of competent cells showed a rise in the mutation frequencies of novobiocin resistance of the order of several hundredfold. Also we performed experiments using the UV-irradiated DNA either sonicated or DNase-treated, which allowed us to propose that such rise in mutation frequency is probably due to the integration of DNA carrying premutagenic photoproducts to the recipient cells' genome. We think that the key point was the low temperature at which the DNA was irradiated in order to obtain the mutagenic effects, since it is likely that at -70 degrees C, the main photoproducts are not the cyclobutane dimers, but are the spore photoproducts, which are probably responsible for the damage that leads to mutagenic effects.