Schematic representation of pCYT:L1 (A) and pSEC:L1 (B) plasmids. A DNA fragment encoding L1 (see Materials and methods) was fused in frame with a DNA fragment containing the Usp45 signal peptide (pSEC:L1), derived from the predominant L. lactis-secreted protein. In pCYT:L1, the fragment encoding SPUsp45 is absent. In these plasmids, L1 expression is controlled by the nisin-inducible promoter (P
) and harbors the Usp45 ribosome binding site and the rho-independent trpA transcription terminator (ter)  for clone stability.