Microcin J25 (MccJ25) is a 21-amino aci
d pepti
de inhibitor active against the DNA-
depen
dentRNA polymerase of Gram negative bacteria. Previously, the structure of MccJ25 was reporte
d to be ahea
d-to-tail circle,
cyclo(-G
1GAGHVPEYF
10VGIGTPISFY
20G-). On the basis of biochemical stu
dies, massspectrometry, an
d NMR, we show that this structure is incorrect, an
d that the pepti
de has an extraor
dinarystructural fol
d. MccJ25 contains an internal lactam linkage between the
-amino group of Gly1 an
d the
-carboxyl of Glu8. The tail (Tyr9-Gly21) passes through the ring (Gly1-Glu8), with Phe19 an
d Tyr20stra
ddling each si
de of the ring, sterically trapping the tail in a noncovalent interaction we call a lassoe
dtail.