  We study permutations over the set of $\ell$-grams, that are feasible in the
sense that there is a sequence whose $\ell$-gram frequency has the same ranking
as the permutation. Codes, which are sets of feasible permutations, protect
information stored in DNA molecules using the rank-modulation scheme, and read
using the shotgun sequencing technique. We construct systematic codes with an
efficient encoding algorithm, and show that they are optimal in size. The
length of the DNA sequences that correspond to the codewords is shown to be
polynomial in the code parameters. Non-systematic with larger size are also
constructed.
