The antibiotic microcin J25 (MccJ25) was cleaved by hydrolysis with thermolysin giving a two-chain peptide (MccJ25-Th19) of 10 and 9 amino acid residues. MccJ25-Th19 with deep modifications in P-hairpin region had no effect on Escherichia coli growth, but still inhibited RNA polymerase in vitro and oxygen consumption in Salmonella strains. MccJ25-ThN showed antibiotic activity on E coli transformed with plasmids containing either fhuA or sbmA genes, which code for proteins involved in MccJ25 transport. These results suggest that an intact beta-hairpin region is crucial for MccJ25 import but not for inhibition of E. coli RNA polymerase or oxygen consumption in Salmonella strains. (C) 2004 Elsevier Inc. All rights reserved.