Evolutionary coorigin of the RNA polymerase sigma factor and the lambda repressor inferred from gene nucleotide sequence homologies.

The lambda phage cI gene and E. coli rpoD gene encoding the lambda repressor and sigma factor, respectively, were aligned with each other based on the internal homologies found in the rpoD gene. Statistical evaluations for these intragenic and intergenic base sequence homologies in the corresponding alignments have conclusively demonstrated that the rpoD gene must have evolved by repeated gene duplications from a primitive gene closely similar to and co-ancestral to the cI gene.