Constitutive and basal secretion from the endocrine cell line, AtT-20

The pRSV trypsinogen expression vector was as described previously (Burgess et al., 1987) . Other expression vectors containing genomic clones carrying their endogenous polyadenylation signals were constructed in a derivative of an RSV-LTR plasmid containing the chloramphenicol acetyltransferase (CAT) gene (Gorman et al., 1982) . In these vectors, the CAT gene was removed and replaced with either a Barn HI or a Hind III site, using DNA linkers, pBR322-MPC-11 K (Kelley et al., 1982) , containing an 8.5-kb Barn HI fragment with the entire rearranged mouse Ig ~ chain genomic clone from the MPC-11 myeloma (Laskov and Scharff, 1970) , was obtained from Dr. Brain Van Ness (University of Iowa, Iowa City, IO). The 8.5-kb K chain genomic clone was cleaved with Pvu II, which removed unwanted 5' sequences and brought the start site of the kappa chain gene within 30 bp of the 5' end of the DNA fragment. This truncated Pvu II-Bam HI fragment was adapted with Barn HI linkers and cloned into an RSV-LTRcontaining expression vector at the Barn HI site. pBR322-human proinsulin, pInsC2 (Laub and Rntter, 1983) , was obtained from Dr. Michael Walker and Dr. William J. Rutter (University of California, San Francisco, CA). The proinsulin gene contained in this plasmid is a chimera between a eDNA at the 5' end and 3' genomic sequences carrying endogenous polyadenylation signals. The proinsulin gene on a 1.2-kb Eco RI fragment was removed from pIasC2 and cloned into an RSV-containing expression vector using Hind HI linkers. All large scale plasmid preps were isolated from HBI01 bacterial cells by the cleared lysate technique (Clewell and Helinski, 1972) and plasmids were purified through two equilibrium gradients of cesium chloride using standard procedures (Maniatis et al., 1982) .