Kenneth D. McMilin

Crosses have been performed which identify phage mutants (chi) which cause recombinational hot spot activity in lambda. The hot spot activity is found in crosses of red(-) gam(-) chi(-) strains in rec(+) hosts; in the crosses reported here, both the chi(-) mutations and the hot spot are located near the right end of the chromosome. The hot spot occurs in standard crosses as well as under conditions which block DNA synthesis, and is dependent on a functional host recB gene.-The chi mutation is shown to be dominant, but the tests do not show whether chi is a gene or a site.

In order to survey the distribution along the bacteriophage lambda chromosome of Rec-mediated recombination events, crosses are performed using conditions which block essentially all DNA synthesis. One parent is density-labeled and carries a genetic marker in the left terminal lambda gene (A), while the other parent is unlabeled and carries a genetic marker in the right terminal lambda gene (R). Both parents are deleted for the lambda recombination genes int and red, together with other recombination-associated genes, by virtue of either (1) a pure deletion or (2) a bio insertion-deletion. The distribution in a cesium density gradient of the resulting A+R+ recombinant phage reflects the chromosomal distribution of the recombination events which gave rise to those phage. Crosses employing either of two different pure deletion phage strains exhibit recombinational hot spot activity located near the right end of the lambda chromosome, between the cI and R genes. This hot spot activity persists when unlimited DNA synthesis is allowed. Crosses employing bio1-substituted phage strains exhibit recombinational hot spot activity located to the right of the middle of the chromosome and to the left of the cI gene. Crosses employing either bio1 or bio69-substituted phage strains indicate that the bio-associated hot spot activity occurs in the presence of DNA synthesis, but is dependent on a functional host recB gene.

The distribution of crossovers along unreplicated chromosomes of bacteriophage lambda has been examined by determining the density distributions and genotypes of particles in the progenies of crosses of density-labeled by ordinary parents in the presence of genetic blocks to replication. The Red and Rec systems combined produce crossovers primarily near the ends (especially the right end) of the chromosome. Removal of the generalized lambda recombination functions by red and gam mutations results in loss of these terminal crossovers; coupled with this loss is a disappearance of the differential dependence of recombination frequencies in terminal and central intervals on DNA synthesis. Removal of the bacterial system by a recA mutation results in severe depression of crossing over among unreplicated phage, with the few recombinants produced by the lambda system occurring near the right end.

Recombination in some intervals of the map of phage lambda is associated with more DNA synthesis than in other intervals. Blockage of DNA synthesis by high temperature in a host temperature-sensitive for DNA synthesis results in the relative reduction of recombinant frequencies in those regions having the larger amounts of recombination-associated synthesis. Reduction of DNA synthesis at normal temperatures by a combination of the bacterial mutation and a mutation in one of the phage genes required for DNA synthesis has the same consequence. Therefore, DNA synthesis enhances recombinant particle formation more in some map intervals than in others.