Dear Amruta

do your mean rosetta (DE3) strain?

of course you can use pet21 with this strain.

you have to use both antibiotics ( ampicillin 100mg/l and chloroanfenicol 34mg/l).

just take in consideration that Rosetta growth much lower than standard bl21(de3)

good luck

Manuele

Thanks Manuele Martinelli . Thank you so much for your reply. Does that mean after the transformation of the pET vector in the host, I should plate the cells on LB media containing both Ampicillin and Chloramphenicol?

Dear Amruta

yes, to be sure that both plasmid are retained into the cells you have to maintain both antibiotic in all the steps, plating, pre-culture and culture.

good luck

Manuele