Regional replication of the bacterial chromosome induced by derepression of prophage lambda. II. Direction and origin.

We have demonstrated previously by DNA-DNA hybridization that induction of lambda phage with wild type O and P genes results in an increase of bacterial DNA in the chromosomal region adjacent to the left of the prophage; is a segment between gal and attlambda (gal DNA) (Imae and Fukasawa, 1970). Evidence is presented in this report that such an increase of bacterial DNA is also seen in the region to the right of the prophage; a segment between bio and attlambda (bio DNA). We postulate therefore that the bidirectional replication of lambda DNA extends beyond the prophage and copies the neighboring host DNA until the prophage is excised. The model is verified by making use of excision-defective lambda phages. The synthesis of gal DNA (or bio DNA) slows down to a halt within 40 min after the induction in the normal lysogens. The results are attributed to the prophage excision: (1) In lysogens for lambdaint, synthesis of the bacterial DNA continues for longer times. (2) The synthesis of the bacterial DNA slows down to a halt in lysogens for lambdaxis or lambdab2 as in the control. However lambda DNA synthesis also slows down in parallel so that the amount of the bacterial DNA relative to that of lambda DNA synthesized by a given time stays constant from 20 min to 80 min. During that time the relative amount of the bacterial DNA rapidly decreases in the normal lysogen.