The role of Chi mutations in the Spi- phenotype of phage lambda: lack of evidence for a gene delta.

Lambda red- gam- makes a small plaque on P2 lysogens (the partial Spi- phenotype). It has been proposed that inactivation of an additional gene delta, mapping in the recombination region, makes the plaque bigger (the full Spi- phenotype) (Zissler et al., 1917b). The present paper demonstrates that the Chi mutation in lambda (Stahl et al., 1975) confers upon red- gam- phage the full Spi- phenotype and that the deletion of the region of the chromosome attributed to delta does not. It appears unnecessary to invoke a gene delta in the Spi- phenotype.