Gp1, the product of one of the essential genes of phi 29 replication, is an RNA binding protein and self-associates to form large complexes. Furthermore, gp1 suppresses the synthesis of phi 29 DNA polymerase and primer protein in the post-transcriptional process. In this report, we have employed seven variants with single amino acid substitutions to analyze the self-assembly of gp1. Using chemical cross-linking and sedimentation assays, amino acid substitutions within the predicted coiled-coil or hydrophobic region were shown to strongly affect the formation of large complexes, suggesting that these two regions were required for the self-assembly of gp1. The self-association of gp1 was suggested to be necessary for the efficient binding to RNA and the translational repression. (c) 2005 Elsevier Inc. All rights reserved.