A CORRELATION BETWEEN INITIATION OF DNA SYNTHESIS AND CELL DIVISION IN MYXOCOCCUS XANTHUS

Abstract

Chromosome completion is a necessary condition for cell division in Myxococcus xanthus as in many other bacteria. By taking advantage of the unioue life cycle of M. xanthus we have been able to obtain evidence that an additional and earlier linkage exists between DNA replication and cell division in M. xanthus FBmpT. In the presence of nalidixic acid (NAL) glycerol induced myxospores germinate but do not divide (snake formation). In contrast , vegetative cells treated with chloramphenicol (CAP) undergo one subsequent cycle of division in the presence of NAL. This difference is surprising because during both treatments (alycerol and CAP) similar kinetics of cell division and DNA synthesis were obtained (immediate cessation of cell division; about 40 % increase in DNA content). One possible explanation for the failure of germinating myxospores to divide in the presence of NAL is that durina myxospore formation chromosomes are blocked just prior to completion. If this were the case, then cells in which chromosomes were allowed to complete in the presence of CAP and subsequently induced to form myxospores should divide in the presence of NAL. However, such germinating myxospores with completed chromosome do not divide in the presence of NAL. Kinetics of DNA synthesis during myxospores formation following release from amino-acid starvation showed that glycerol induction involves an immediate cessation of new cycles of DNA replication. During germination DNA synthesis started synchronous ly between 3 - 5 hrs, whereas the first round of synchronous cell division appeared between 4 - 6 hrs indicating that the first division follows a short interval of DNA synthesis. Treatment of germinating myxospores with NAL at different time intervals indicated that the critical time period in which DNA synthesis is necessary for the first division beqan at 3.5 hrs. This time period (3.5-5 hrs) was shown to involve only new initiations of DNA replication by the following experiment: Myxospores induced by glycerol in the presence of NAL completed their chromosomes during the first 4 hr of germination. Hence, the potential for DNA elongation is present much earlier than the critical time period mentioned above. It is proposed that the first division following germination depends on DNA initiation. This requirement is not the synthesis of DNA per se,but involves the synthesis of specific RNA and protein triggered by DNA initiation: when germinating myxospores were released from inhibition of DNA synthesis by removal of NAL at 6 hrs, they failed to divide in the presence of CAP or rifampicin. These data support the hypothesis that during normal growth a component necessary for cell division is synthesized during initiation of DNA replication. This hypothetical component is necessary for subsequent cell division. During myxospore formation this component is destroyed.