Polyribonucleotide analogs which are partially substituted by the introduction of mercapto groups and fluorine at the 5-position of some of their cytosine and uracil bases possess effective in vivo antitumor activity. Double stranded complexes formed from such polynucleotides are especially active as interferon inducers.

This involves an in vitro assay system to screen for mutagenic and/or carcinogenic agents based on alterations in the fidelity of DNA synthesis. In the system, chemicals will be tested for their mutagenicity or carcinogenicity by measuring increases in the number of mistakes incorporated by purified DNA polymerases using synthetic polynucleotide templates. This system offers the advantage of performing this analysis in the test tube so that all parameters in the reaction can be monitored. Agents that enhance misincorporation during DNA synthesis would be predicted to have a higher probability of being mutagens or carcinogens in the human population.

The invention relates to a novel process for the production of (2'-5')A(pA).sub.n pA chains wherein n is an integer of from 1 to 6, which comprises polymerizing adenosine (2'-3') monophosphate to obtain a mixture of 2'-5'- and 3'-5'-polyadenylic acid, treating the mixture with ribonuclease P.sub.1 to cleave selectively the 3'-5'-bonds so as to retain the 2'-5'-polymeric moieties with a terminal 5'-phosphate moiety and removing the terminal phosphate groups by means of alkaline phosphatase. The mixture may be purified by separation on an anionic ion exchanger and collecting the individual fractions. The invention further relates to pharmaceutical compositions for inhibiting the immune response of mammals which comprise as active ingredient a (2'-5')oligo-isoadenylate defined above. The preferred compositions are those with a preponderance of the trimer.

Enzymatic synthesis of oligonucleotides may be performed in a single vessel without intermediate purification, by the steps of: (a) combining a nucleotide primer sequence and a blocked nucleotide in the presence of a chain extending enzyme whereby a reaction mixture is formed containing the blocked nucleotide coupled to the nucleotide primer sequence at its 3' end; (b) inactivating the chain extending enzyme; (c) removing the blocking group from the primer-blocked nucleotide to form a primer-nucleotide product; and converting any unreacted blocked nucleotide to an unreactive form which is substantially less active as a substrate for the chain extending enzyme than the blocked nucleotide.

Enzymatic synthesis of a repeat region of an oligonucleotide may be performed by the steps of: (a) combining a primer and a blocked nucleotide in the presence of a chain extending enzyme whereby a primer-blocked nucleotide product is formed containing the blocked nucleotide coupled to the primer at its 3'-end; (b) removing the blocking group from the 3'-end of the primer-blocked nucleotide product using a 3'-phosphatase enzyme substantially without removing the 3'-phosphate blocking group from unreacted 3'-phosphate-blocked nucleotide; and (c) repeating the cycle of steps (a) and (b), using the primer-nucleotide product of step (b) as the primer for step (a) in the next cycle, for sufficient cycles to form the oligonucleotide product. These cycles are performed preferably in a single vessel without intermediate purification of oligonucleotide product. Also disclosed is a process for synthesizing an oligonucleotide having a defined sequence including at least one repeat region and one non-repeating region, wherein at least one non-repeating region is synthesized by reaction cycles using the steps of extending a primer with a 3'-blocked nucleotide, inactivating unreacted 3'-blocked nucleotide, and removing the blocking group from the extended primer. The disclosed processes may be used to synthesize repeat regions of oligoribonucleotides.