Studies on bacterial growth and replication.
2015-11-19T08:53:06Z (GMT) by
Previous work in this laboratory has indicated that in thymine low requiring strains of Escherichia coli, the transit time of a replication fork can be varied without an appreciable effect on the growth rate, simply by varying the thymine concentration in the growth medium. This results in a change in the DNA concentration and therefore changes the dosage of most genes. I have defined three more precise parameters which have previously been encompassed in the term gene dosage. These I have called gene concentration, relative gene dosage, and gene dosage. On theoretical grounds, these three parameters change in separate and distinct ways as a function of the replication velocity and as a function of the relative position of the gene on the chromosome. In order to distinguish the relative importance of each of them in determining the overall rate of protein synthesis, I have compared the rate of enzyme synthesis from five genes in a fully repressed or induced state and one in an autogenous state in cultures growing at the same rate but having different transit times. The data permit the following conclusions: (1) The initiation mass is independent of the transit time of a replication fork at one growth rate. (2) Replication is bidirectional. The chromosome origin is near the gene coding for tryptophanase and the terminus near the gene specifying tryptophan synthetase. The data place the origin in the region of 67 minutes on the E.coli linkage map. (3) The output of a gene under conditions when it is subject to regulation by a cytoplasmic effector decreases as its concentration is increased. In addition, I have used the observed differential effect of changes in replication velocity on gene concentration to determine if the replication of the sex factor F is directly coupled to any stage of the chromosome replication cycle. The data preclude the possibility that F replication is coupled to any part of the chromosome replication cycle other than to initiation (or to the replication of genes close to the origin) or, at the growth rate used here, that it occurs at a constant cell mass.