Genetic Ultrafine Structure in the T4rii Region.

T H E frequency of genetic recombination in phage crosses is often taken as a measure of the physical separation between the mutated nucleotide sites; the smaller the recombination frequency, the smaller the separation. Assuming the nucleotide is an indivisible molecular unit of heredity, a lower limit to the recombination frequencies between point mutants might be expected, corresponding to mutations at neighboring nucleotide sites. BENZER (1957) defined a physical unit of recombination, the recon, as “the smallest element in the one-dimensional array that is interchangeable (but not divisible) by genetic recombination.” At that time, 0.02 percent was about the smallest non-zero recombination frequency experimentally encountered in phage T4. From the DNA content of T4 and the size of the rZZ region relative to the whole genetic map, it was inferred that the size of the recon corresponding to a recombination frequency of 0.02 percent would not be larger than approximately two nucleotide pairs. However, this paper will show that there is no evidence for a lower limit to the frequency of recombination between two parental types containing single mutations at different sites. Furthermore, it will be seen that very small recombination frequencies cannot be used reliably to measure nucleotide distances o r the size of the recon. The experiments are based on sensitive recombination tests, which can detect fewer than one recombinant in lo8 parental types. At frequencies below approximately 1 O-’ percent, aberrant genetic properties are observed. The resolution of the phage genetic map by recombination frequencies less than lo-’ percent is called ultrafine structure. A brief report of this work has been presented (TESSMAN 1963).