4.7 GenomeeReducing Efforts and the Immact of Streamlining 65recombinogenic or mobile DNA stabilized the MDS genomes and provided a
host free of IS contamination for plasmid preparations and gene libraries,
reducing the chances for cloning artifacts, solving a frequently arising but usu-
ally overlooked problem. Many cryptic virulence genes were also removed, pre-
sumably increasing the safety of the strains. High yields of recombinant protein
production were achieved in MDS cells. Genome reduction also led to unantici-
pated beneficial properties: high electroporation efficiency and accurate propa-
gation of recombinant genes and plasmids with strong secondary structure that
were unstable in other strains. It was demonstrated that the stability of lentiviral
vectors containing long direct repeats was significantly enhanced in MDS42
[99]. Genome stability was further increased by deleting the three SOS-
inducible, error-prone DNA polymerases PolII, PolIV, and PolV [4], signifi-
cantly reducing point- mutation rates, thereby allowing more faithful expression
E. coli
K-12OriTer200015002500300035004000450050010008 7 6 5 4 3 2 1Figure 4.4 Deletion map of reduced-genome E. coli strains. Rings depict features mapped to
the genome of E. coli K-12 MG1655, numbered on the perimeter in kilobase pair. Outward from
the center, (1) strain-specific K-12 genomic islands longer than 4 kbp [96], (2) essential genes
(www.shigen.nig.ac.jp/ecoli/pec/index.jsp), and (3)–(8) set of deletions constructed by
Goryshin et al. [43], Yu et al. [73], Hashimoto et al. [97], Pósfai et al. (MDS42: black boxes, MDS69:
black and gray boxes) [29, 85], Mizoguchi et al. (MGF-01) [47], and Hirokawa et al. (DGF-298)
[98], respectively. Ori and ter indicate the origin and terminus of replication, respectively.