Studies have continued on the various steps involved in the biosynthesis of putrescine and spermidine in Escherichia coli, using a variety of genetic and enzymological techniques. Mutants have been made in the various genes, and recombinant plasmids have been constructed that overproduce the biosynthetic enzymes. Special attention has been directed towards the study of S-adenosylmethionine decarboxylase, both because it is a key enzyme in the biosynthesis of spermidine and because it has a very special structural and enzymological characteristics; namely, adeonsylmethionine decarboxylase contains covalently-bound pyruvate as an essential part of the active site. We have sequenced the gene and, after insertion of the gene into an expression vector, have demonstrated that the enzyme (subunit molecular weight 17,000) is first synthesized as a proenzyme (subunit molecular weight 36,000).