Efficient production of eukaryotic proteins in E. coli may require all of the following except?

A) altered codon usage within the heterologous sequences to approximate the codon bias in E. coli
B) the use of cDNAs, that are free of introns, as eukaryotic transgenes
C) an E. coli expression vector with a promoter sequence that binds RNA polymerase
D) an E. coli expression vector with a Shine- Dalgarno sequence for efficient translation
E) a negative selectable marker to select against nonhomologous recombination events