Single-stranded end preserving adaptors

A kit comprising:

• a) a plurality of single-stranded adaptors, wherein each of said single-stranded adaptors comprises:
• i) a nucleic acid sequence comprising a 5′ end and a 3′ end and ii) optionally a 3′ end blocking group attached to said 3′ end of said nucleic acid sequence, wherein said 3′ end blocking group is optionally a modified non-canonical nucleotide or a phosphate group,
• wherein said 5′ end of said nucleic acid sequence is adenylated, and
• wherein said nucleic acid sequence further comprises:
• a 5′ region, a 3′ region, and a cleavable region between said 5′ and 3′ regions, wherein said cleavable region comprise a nucleic acid backbone linkage, and is optionally methylated at every, or nearly every, cytosine present in said nucleic acid sequence;
• b) a single strand ligase that is a thermostable lysine-mutant ssDNA/RNA ligase which is a mutated version of a precursor thermostable ssDNA/RNA ligase that has a Motif I EKx(D/N/H)G,
• wherein said thermostable lysine-mutant replaces lysine (K) in said Motif I with an amino acid selected from alanine (A), serine(S), cysteine (C), valine (V), threonine (T), and Glycine (G), and
• wherein replacement of said lysine (K) in said Motif I results in said thermostable lysine-mutant ssDNA/RNA ligase to have step 3 ligase activity, but not step 2 adenylyl transfer activity that is present in said precursor thermostable ssDNA/RNA ligase, and
• c) an RtcB enzyme.