TABLE 4.
Yeast SRB/mediator subunitsa
| Factor | Mass (kDa) | Gene(s) | Essential | Metazoan homologs | Reference(s) |
|---|---|---|---|---|---|
| Srb2 | 23 | SRB2 | No | 252, 260, 261, 483 | |
| Srb4 | 78 | SRB4 | Yes | 252, 261, 483, 484 | |
| Srb5 | 34 | SRB5 | No | 252, 261, 483 | |
| Srb6 | 14 | SRB6 | Yes | 252, 261, 483, 484 | |
| Srb7 | 16 | SRB7 | Yes | Yes | 194, 283 |
| Srb8 | 167 | SRB8, SSN5, ARE2 | No | 194, 283 | |
| Srb9 | 160 | SRB9, SSN2, UME2 | No | 194, 283 | |
| Srb10 | 63 | SRB10, SSN3, UME5, ARE1 | No | 265, 285, 461, 516 | |
| Srb11 | 36 | SRB11, SSN8, UME3 | No | 98, 265, 285 | |
| Gal11 | 38 | GAL11, SPT13, SDS4, RAR3 | No | 130, 252, 261 | |
| Sin4 | 111 | SIN4, SSN4, TSF3 | No | 72, 230, 282 | |
| Rgr1 | 123 | RGR1 | Yes | Yes | 230, 282 |
| Rox3b | 25 | ROX3, SSN7 | Yes | 170, 396 | |
| Med1 | 64 | MED1 | No | Cited in 330 | |
| Med2 | 48 | MED2 | No | 330 | |
| Med3, Pgd1, Hrs1 | 47 | PGD1, HRS1 | No | 330 | |
| Med4 | 32 | MED4 | Yes | 330 | |
| Med6 | 33 | MED6, MTR32 | Yes | Yes | 278 |
| Med7 | 32 | MED7 | Yes | 330 | |
| Med8 | 25 | MED8 | Yes | 330 |
Srb2, Srb4, Srb5, Srb6, and Srb7 are subunits of the SRB/mediator as described by both the Kornberg and Young laboratories. Srb8 to Srb11 are components of the SRB/mediator defined by Young and coworkers (194) but not by Kornberg and coworkers (330). The SWI/SNF complex has also been reported by Young and colleagues to be associated with the holoenzyme (535) but is not included in the SRB/mediator defined by Kornberg and coworkers (330). Sug1 was identified as a suppressor of a gal4 mutation (468) and proposed to be a component of SRB/mediator (252, 261, 469). More recently, Sug1 has been defined as a subunit of the 26S proteosome (402) and is no longer included as a subunit of the SRB/mediator (330).
Rox3 was initially reported to be identical to Med8 (170). Subsequent analysis of Med8 revealed two distinct polypeptides, one of which is Rox3; however, the Med8 designation was retained for the other (330). Accordingly, Rox3 and Med8 now refer to distinct polypeptides that are both components of SRB/mediator.