Assembly of a yeast 5 S RNA gene transcription complex.

The assembly of the yeast transcription factors IIIA, IIIB, and IIIC, represented by crude chromatographic fractions, into a stable complex on the 5 S RNA gene has been assessed using a template competition assay (Bogenhagen, D. F., Wormington, W. M., and Brown, D. D. (1982) Cell 28, 413-421). The yeast factor A can interact transiently with the yeast 5 S RNA gene, but both factors A and C are required for the formation of a stable complex. Factor A, but not factor C, is sequestered into this complex. Both factors A and C, however, are required for the sequestration of factor B into the 5 S RNA gene transcription complex. During this study, it was noted that an amount of factor C which was insufficient to lead to significant transcription of the 5 S RNA gene could nonetheless efficiently stabilize factor A on the template. This suggests that factor C may initially act catalytically to stabilize a transient interaction of factor A with the 5 S RNA gene before assuming a stoichiometric role in the assembly of the complete transcription complex. Alternative interpretations of this observation are discussed.