Direct plasmid transfer from replica-plated E. coli colonies to competent B. subtilis cells. Identification of an E. coli clone carrying the hisH and tyrA genes of B. subtilis.

We have cloned the hisH tyrA wild-type genes of Bacillus subtilis with the aid of the chimeric plasmid pBJ194, which replicates both in B. subtilis and Escherichia coli. Primary cloning was done in E. coli. The original E. coli clone, carrying the recombinant plasmid (pGR1) which complements hisH tyrA mutants of B. subtilis, was selected directly from a mixture of plated E. coli clones by replicaplating these clones onto minimal agar plates without tyrosine, spread just before with competent B. subtilis cells. After overnight incubation clusters of small colonies had developed exclusively in the E. coli [pGR1] colony prints. The Tyr+ minicolonies were shown to be B. subtilis carrying pGR1 because (i) their appearance depended linearly on the number of B. subtilis cells plated, (ii) they produced extracellular protease and amylase and (iii) plasmids could be reisolated from the minicolonies and used to transform B. subtilis recE4 tyrA1 both to Cmr and Tyr+. Plasmid pGR1 transfer through replica plating was compared with plasmid transfer in liquid. Both systems depended on transformable B. subtilis strains and were sensitive to DNAseI. However, whereas integration of the tyrA+ gene into the chromosome and concomitant loss of plasmids occurred frequently during regular plasmid transformation of Rec+ B. subtilis, this was a rare event during plasmid transfer through replica plating.