A comparison of the number of nucleotides per unit length in Escherichia coli and phage T4 chromosomes.

The grain density produced in radioautographs by fully labeled bacteriophage T(4) chromosomes was compared with the grain densities produced by fully labeled and half-labeled exponential phase Escherichia coli chromosomes. Taking into account the differing molar proportions of thymidine in E. coli and T(4) chromosomes, it was seen that the two types of chromosomes have approximately equal numbers of nucleotides per unit length, indicating that their structures are the same. Using molecular weight estimates in the literature for the T(4) and related T(2) chromosomes, and the lengths obtained in the radioautographs, the T(4) chromosome was estimated to have between 1.9 and 3.6 nucleotides/3.4 A. On the basis of these values alone, the E. coli and T(4) chromosomes could be either one Watson-Crick helix in a form about equal to or more condensed than the B form, or two helices more stretched out than the B form. The length of the T(4) chromosome was 48.7 +/- 4.1 mu when dried on the dull side of Millipore membranes and 42.3 +/- 4.8 mu on the shiny side, under the conditions used. Thus, the supporting surface apparently affects the configuration of a chromosome. Further evidence is also presented in support of the conclusion that the E. coli chromosome undergoes semi-conservative replication.