Directed node-label controlled graph grammars (DNLC grammars) are sequential graph rewriting systems. In a direct derivation step of a DNLC grammar a single node is rewritten. Both the rewriting of a node and the embedding of a "daughter graph" in a "host graph" are controlled by the labels of nodes only. We study the use of those grammars to define string languages. In particular we provide a characterization of the class of context-free string languages in terms of DNLC grammars.