My tests of UV sensitivity have found something odd about strain DH5alpha. I know it's supposed to be recA1, but both our standard lab strain and an independent DH5alpha derivative obtained from a European lab are no more UV-sensitive than the Rec+ strains we've tested (W3110, C600, BW25113), and much more UV-resistant than our NM554 strain (recA13).
This suggests that our DH5alpha strains are not really RecA-. That would be consistent with the RA's results in her transformation assays, but it seems unlikely that both our stocks are not what they're supposed to be.
Another weird result is that when DH5alpha is carrying a low-copy sxy expression plasmid it becomes as UV-sensitive as NM554. But the UV-sensitivity of the Rec+ strain BW25113 isn't altered by the same plasmid. (sxy expression in these cells wasn't induced with IPTG, but that may not matter because DH5alpha is deleted for lacZYA and maybe also lacI.)
I guess I should check other components of the genotype of our DH5 alpha strains before I start emailing the RecA experts for advice. It's supposed to be F-, φ80dlacZΔM15, Δ(lacZYA-argF) U169, deoR, recA1, endA1, hsdR17(rk-, mk+), phoA, supE44, λ-, thi-1, gyrA96, relA1. Easy to check for Lac- (the RA just made some MacConkey plates), but many E. coli strains are Lac- so that's not very diagnostic. Hmmm....
Sixty-four years later: How Watson and Crick did it
21 hours ago in The Curious Wavefunction