Abstract A broad-host-range promoter-probing vector, pMY3 (8.0 kb), was constructed for cloning of DNA fragments containing promoter sequences ofXanthomonas campestrispv.campestris.This vector (pMY3) consists of the RK2 replicon, promoterlessluxABgenes, thethrattenuator to block the transcription of RNA into theluxABregion, and multiple cloning sites for cloning of the fragment carrying promoter sequences. The feasibility of using pMY3 as a promoter-probing vector in bothE. coliand Xc17 was demonstrated by using thelacpromoter ofE. coli,and theamypromoter ofX. campestrisin Xc17. Among the 63 promoter-containing fragments cloned from Xc17, only 9 were able to express inE. coli.It appears thatX. campestriscan recognize mostE. colitype promoters, but,E. colican recognize only a small portion of theX. campestristype promoters.