In contrast to the extensive understanding of the zinc finger-DNA interactions, less is known about zinc finger-zinc finger interactions. GATA-1 and Sp1 are transcription factors with zinc finger domains for DNA binding. The interaction between the GATA-1 and Sp1 zinc finger domains is important for synergistic transcriptional effects in erythroid genes. Despite the biological importance of the GATA-1 and Sp1 interaction, the molecular mechanism of the interaction remains unclear. We constructed a series of deletion mutants of the zinc finger domains of GATA-1 and Sp1 to identify the regions within the GATA-1 and Sp1 zinc finger domains that interact. The zinc finger-zinc finger interaction modes were also estimated from calorimetric measurements. This revealed that the interaction between the Sp1 and GATA-1 zinc finger domains was primarily electrostatic, and that the linker region of the Sp1 zinc fingers is important for the association with the GATA-1 zinc finger domains. We propose a new molecular mechanism for zinc finger-zinc finger interactions that should contribute to our understanding of the bio-functional role of the interaction between GATA-1 and Sp1. (c) 2010 Elsevier Inc. All rights reserved.