I have a custom plasmid which expresses mCherry and will act as a reporter. I can transform this plasmid into DH5a and BL21. With DH5a I get a good number of singular colonies, but with BL21 I get a few thick, white colonies which do not grow in liquid LB. The DH5a does grow in LB. Does anyone have an idea as to why this is?
The efficiency of transformation in DH5a cells is much higher as compared to BL21 cells. I work with both of them and over the years I have seen that DH5a produces many small colonies as compared to BL21 which produces thick colonies fewer in number. However, its weird that you are getting BL21 colonies in LB agar while no growth in liquid LB.
Sometimes BL21 does not grow at all in liquid culture after transformation and especially if the expression vector does not encode lacI. This is likely because T7 expression in BL21 is not tightly regulated and as such toxic proteins may affect cell viability. Simply use a strain expressing lysY.
Is the strain you are using BL21 or BL21(DE3) and is mCherry transcribed from T7 promoter? The question is whether you are having some strain effect or whether you just have high level expression of mCherry and this is generating some toxicity. It could also be that you are just not getting transformants with BL21 and those few colonies are some sort of contaminants, have you done a control transformation with some neutral plasmid?
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