During the final stages of phi X174 morphogenesis, there is an 8.5-A radial collapse of coat proteins around the packaged genome, which is tethered to the capsid's inner surface by the DNA-binding protein. Two approaches were taken to determine whether protein-DNA interactions affect the properties of the mature virion and thus the final stages of morphogenesis. In the first approach, genome-capsid associations were altered with mutant DNA-binding proteins. The resulting particles differed from the wild-type virion in density, native gel migration, and host cell recognition. Differences in native gel migration were especially pronounced. However, no differences in protein stoichiometries were detected. An extragenic second-site suppressor of the mutant DNA-binding protein restores all assayed properties to near wild-type values. In the second approach, phi X174 was packaged with foreign, single-stranded, covalently closed, circular DNA molecules identical in length to the phi X174 genome. The resulting particles exhibited native gel migration rates that significantly differed from the wild type. The results of these experiments suggest that the structure of the genome and/or its association with the capsid's inner surface may perform a scaffolding-like function during the procapsid-to- virion transition.