Dispersal of the parental nucleic acid of bacteriophage T4 among its progeny

The distribution of the parental deoxyribonucleic acid (DNA) passed on from T4 bacteriophages to their offspring has been measured by allowing radioactive decay to proceed in first generation progeny phages containing radiophosphorus atoms transferred to them from highly 32 P-labeled parental viruses. The amount of once-transferred parental radioisotope transferable from the first generation progeny to their own second-generation progeny was then measured at various stages of radioactive decay, in order to determine the lethal effects produced by 32 P disintegrations within those first generation progeny particles which carry the parental atoms. It was found that whereas before decay about half of the parental 32 P harbored by the first generation progeny can be once more transferred to the second generation progeny, subsequent radioactive disintegrations in the first generation progeny rapidly reduce the transferable radio-isotope to one-third of the initial value. This final third of the parental 32 P remains transferable, however, even after further extensive radioactive decay has been allowed to take place. Quantitative evaluation of the observed rate of loss of transferability with decay indicates that approximately 60% of the transferred parental DNA appears in the first generation progeny in aggregates each amounting to 15% of the total DNA complement per phage and that the remainder is dispersed into small fragments no greater than 0·3% of the amount of DNA per phage. This dispersal of the parental nucleic acid does not seem to be produced by random breakage forces acting on the DNA in the course of intracellular phage reproduction. For it was found that the distribution of the parental isotope among the first generation progeny remained unaffected if the bacteria infected with the 32 P-labeled parent phages were temporarily exposed to chloramphenicol. Under these conditions, phage DNA synthesis proceeds while formation of intact progeny phages is held in abeyance until removal of the antibiotic. Hence if the viral DNA were subject to the action of random breakage forces during its replication, this artificial extension of the eclipse should have caused a greater dispersal of the transferred atoms. The distribution of parental isotope, furthermore, was also studied among the second generation progeny by carrying out third generation transfer experiments with aliquots of second generation progeny at various stages of radioactive decay. It was found that the transferable fraction of the twice-transferred parental isotope decreased with radioactive decay in the same manner as that of the one-transferred label in the first generation progeny. This indicates that after a second transfer cycle the distribution of the parental atoms remains essentially unchanged, in contrast to the anticipated further dispersal if random breakage forces obtained. The observed distribution of the transferred DNA atoms must, therefore, reflect an a priori division of the nucleic acid of T-even phages into parts which are destined to be transferred in large and small fragments respectively. These results are in agreement with those obtained by Levinthal (1956) through direct autoradiography.