The mobilizable shuttle cloning vectors, pAT18 and pAT19, are composed of: (i) the replication origins of pUC and of the broad-host-range enterococcal plasmid pAM beta 1; (ii) an erythromycin-resistance-encoding gene expressed in Gram- and Gram+ bacteria; (iii) the transfer origin of the IncP plasmid RK2; and (iv) the multiple cloning site and the lacZ alpha reporter gene of pUC18 (pAT18) and pUC19 (pAT19). These 6.6-kb plasmids contain ten unique cloning sites that allow screening of derivatives containing DNA inserts by alpha-complementation in Escherichia coli carrying the lacZ delta M15 deletion, and can be efficiently mobilized by self-transferable IncP plasmids co-resident in the E. coli donors. Plasmids pAT18, pAT19 and recombinant derivatives have been successfully transferred by conjugation from E. coli to Bacillus subtilis, Bacillus thuringiensis, Listeria monocytogenes, Enterococcus faecalis, Lactococcus lactis, and Staphylococcus aureus at frequencies ranging from 10(-6) to 10(-9). The presence of a restriction system in the recipient dramatically affects (by three orders of magnitude) the efficiency of conjugal transfer of these vectors from E. coli to Gram+ bacteria.