Understanding the Differences between Genome Sequences of Escherichia coli B Strains REL606 and BL21(DE3) and Comparison of the E. coli B and K-12 Genomes

F. William Studier, Patrick Daegelen, Richard E. Lenski, Sergei Maslov, Jihyun F. Kim

Research output: Contribution to journalArticlepeer-review


Each difference between the genome sequences of Escherichia coli B strains REL606 and BL21(DE3) can be interpreted in light of known laboratory manipulations plus a gene conversion between ribosomal RNA operons. Two treatments with 1-methyl-3-nitro-1-nitrosoguanidine in the REL606 lineage produced at least 93 single-base-pair mutations (∼ 90% GC-to-AT transitions) and 3 single-base-pair GC deletions. Two UV treatments in the BL21(DE3) lineage produced only 4 single-base-pair mutations but 16 large deletions. P1 transductions from K-12 into the two B lineages produced 317 single-base-pair differences and 9 insertions or deletions, reflecting differences between B DNA in BL21(DE3) and integrated restriction fragments of K-12 DNA inherited by REL606. Two sites showed selective enrichment of spontaneous mutations. No unselected spontaneous single-base-pair mutations were evident. The genome sequences revealed that a progenitor of REL606 had been misidentified, explaining initially perplexing differences. Limited sequencing of other B strains defined characteristic properties of B and allowed assembly of the inferred genome of the ancestral B of Delbrück and Luria. Comparison of the B and K-12 genomes shows that more than half of the 3793 proteins of their basic genomes are predicted to be identical, although ∼ 310 appear to be functional in either B or K-12 but not in both. The ancestral basic genome appears to have had ∼ 4039 coding sequences occupying ∼ 4.0 Mbp. Repeated horizontal transfer from diverged Escherichia coli genomes and homologous recombination may explain the observed variable distribution of single-base-pair differences. Fifteen sites are occupied by phage-related elements, but only six by comparable elements at the same site. More than 50 sites are occupied by IS elements in both B and K, 16 in common, and likely founding IS elements are identified. A signature of widespread cryptic phage P4-type mobile elements was identified. Complex deletions (dense clusters of small deletions and substitutions) apparently removed nonessential genes from ∼ 30 sites in the basic genomes.

Original languageEnglish (US)
Pages (from-to)653-680
Number of pages28
JournalJournal of Molecular Biology
Issue number4
StatePublished - Dec 11 2009
Externally publishedYes


  • CP4-type mobile elements
  • E. coli B genome
  • SNP distribution
  • UV deletions
  • complex deletions

ASJC Scopus subject areas

  • Biophysics
  • Structural Biology
  • Molecular Biology


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