What substrate does S1 nuclease act upon?

S1 nuclease is an enzyme that specifically targets single-stranded nucleic acids, which includes both single-stranded DNA and single-stranded RNA. This enzyme acts by cleaving the phosphodiester bonds in the backbone of these nucleic acids, thus dephosphorylating the single-stranded regions.

Option C accurately describes the substrate specificity of S1 nuclease, as it can effectively act upon both types of single-stranded nucleic acids. This is particularly useful in various molecular biology applications, such as analyzing RNA transcripts, studying DNA-RNA hybrids, and preparing samples for further downstream applications, like cloning or sequencing.

In contrast to this, double-stranded DNA and circular DNA molecules are not substrates for S1 nuclease. The enzyme is designed to recognize and act upon single-stranded conformation, which is not present in double-stranded structures. Therefore, single-stranded nucleic acids remain the primary substrates for S1 nuclease activity.