The biochemical and physicochemical properties of DNA oligomers containing phosphorodithioate linkages in various configurations were evaluated. Duplex stability studies, which were carried out by thermal denaturation analysis with complementary unmodified DNA, indicated a highly cooperative process similar to completely unmodified duplexes. Oligomers containing phosphorodithioate linkages were found to have reduced melting temperatures relative to unmodified duplexes, with the degree of Tm depression paralleling the percent phosphorodithioate composition of the oligomer. Relative to activation of RNase H, DNA oligomers containing up to 50% phosphorodithioate linkages were able to direct RNase H degradation with the same efficiency as unmodified DNA while those containing from 50 to 100% acted with somewhat reduced efficiency. At limiting concentrations, an oligomer containing alternating phosphorodithioate and phosphate linkages was able to direct RNase H degradation of the target RNA in an extended incubation, while an unmodified oligomer did not. The nuclease resistance of phosphorodithioate-containing oligomers was evaluated in HeLa cell nuclear and cytoplasmic extracts, in human serum, and with nucleases S1 and DNase I. Oligomers containing alternating phosphorodithioate and phosphate were highly resistant to degradation in all systems. However, oligomers having more than one unmodified linkage separating phosphorodithioates were degraded rapidly by DNase I, while demonstrating stability to degradation in all other systems tested. These results indicate that phosphorodithioate-containing DNA oligomers are highly nuclease-resistant, are able to form stable duplexes with complementary nucleic acid sequences, and efficiently direct RNase H degradation of target RNA.

A radioimmunoassay has been developed for peptides containing the carboxyl-terminal sequence Arg-Phe-NH2 (RFamide). Using this radioimmunoassay and applying cation-exchange chromatography and HPLC, we (2,3,8,9). This paper describes the development of a radioimmunoassay for the coelenterate RFamide-like peptides and the purification and sequencing of such a peptide from the sea anemone Anthopleura elegantissima. METHODSRadioimmunoassay. Four rabbits were immunized with RFamide (Bachem, Bubendorf, Switzerland) that was coupled by carbodiimide to bovine thyroglobulin (Sigma). The antisera were treated overnight at 4°C with bovine thyroglobulin at 1 mg/ml, followed by centrifugation, to remove antibody populations specific for the carrier protein. One antiserum (code 145IV) was selected for the present study.125I-labeled Tyr-Phe-Met-Arg-Phe-NH2 was used as a tracer in the radioimmunoassay. It was prepared by adding 5 1.d (0.5 mCi; 18 MBq) of Na125I (Amersham) to 15 ,ul of 0.2 M sodium phosphate buffer, pH 7.0, containing 0.22 ,ug of Tyr-PheMet-Arg-Phe-NH2 (Peninsula Laboratories, San Carlos, CA), followed by addition of 10 ul of phosphate buffer containing 1.4 ,g of chloramine-T. After mixing, the reaction was stopped after 20 sec with 500 1.l of 0.3% Na2S205. The mixture was immediately loaded on a methanol/watertreated Sep-Pak C18 cartridge (Waters Associates), which was quickly rinsed with 40 ml of twice-distilled water, to remove unreacted 1251. Labeled material was eluted with 2-ml portions of 40%, 60%, 80%, and 90% (vol/vol) methanol and three times with 2-ml portions of 100% methanol. Each fraction was collected in 6 ml ofRIA buffer, consisting of0.05 M sodium phosphate at pH 7.4 and 1% bovine serum albumin (Sigma). The major portion of the tracer was eluted at 90% or 100% methanol. The quality of an aliquot (3000 cpm) of the tracer fractions was tested by overnight incubation, at 40C, in

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