On the power of P systems with DNA-worm-objects

We introduce a variant of P systems with string-objects - called worm-objects - inspired in the DNA computing area. These systems work with multisets; of string-objects processed by splitting, mutation, replication and recombination. This model is simpler (we eliminate the replication operation) and more realistic (the recombination operation is changed by the simpler one of suffix-prefix or head-tail concatenation developed in the DNA computing framework) than the previous one. The result of a computation is the set of strings sent out of the system. We work with multisets of strings but we generate languages instead of sets of numbers. We prove that, without priority among rules or other control mechanisms, (1) these P systems with at most three membranes can generate all recursively enumerable languages, (2) with non-decreasing length mutation and splitting rules, three membranes are enough to generate the family of context-sensitive languages, and (3) with these restricted types of splitting and mutation rules, four membranes can generate the family of recursively enumerable languages.