Unlike the core subunits, RPB4 and RPB7 subunits are much smaller with protein masses of 25kDa and 19kDa, respectively. SDS-PAGE analysis demonstrates that RPB4 and RPB7 have molecular weights of 32kDa and 17kDa, respectively. Both subunits may contribute to fine-tuning the transcription apparatus (Kim 1989). These subunits are not essential for yeast cell viability at moderate temperatures, as their growth rates are essentially equivalent to that of wild type. When subject to heat shock, cells lacking RPB4 rapidly loss their ability to transcribe genes and experience a dramatic loss in function (Kim 1989). Therefore, although not required for viability, RPB4 is essential for efficiency (Young 1991). When cells lacking RPB4 are subject to the nutrient stress, such as that accompanying entrance into the stationary phase, they exhibit a substantial decline in mRNA synthesis and viability relative to wild type cells (Azuma 1991).
The RPB4 and RPB7 subunits of yeast RNA polymerase II form a heterodimeric complex essential for promoter directed transcription initiation (Cheney 2001; Kim1989). A mutant yeast strain lacking the RPB4 gene and RPB7 subunit was isolated by Woychik and Young in 1989. This mutant strain is indistinguishable from the wild type with respect to chain elongation and promoter-independent transcription initiation, but activity is greatly reduced during promoter-directed transcription initiation in the mutant (Young 1991). In addition, electron crystallography demonstrates that yeast polymerase lacking these subunits favor the closed conformation of RNA polymerase II (Kornberg 1997). This further supports the belief of researchers that RPB4 and RPB7 play roles in transcription initiation, probably after polymerase-DNA interaction (Kornberg 1997).