The RNA polymerase II of Saccharomyces cerevisiae exists in holoenzyme forms containing a complex, known as the mediator, associated with the carboxyl-terminal domain. The mediator includes several SRB proteins and is required for transcriptional activation. Previous work showed that a cyclin-dependent kinase-cyclin pair encoded by SSN3 and SSN8, two members of the SSN suppressor family, are identical to two SRB proteins in the mediator. Here we have identified the remaining SSN genes by cloning and genetic analysis. SSN2 and SSN5 are identical to SRB9 and SRB8, respectively, which encode additional components of the mediator. Genetic evidence implicates the SSN genes in transcriptional repression. Thus, these identities provide genetic insight into mediator and carboxyl-terminal domain function, strongly suggesting a role in mediating transcriptional repression as well as activation. We also show that SSN4 and SSN7 are the same as SIN4 and ROX3, respectively, raising the possibility that these genes also encode mediator proteins.Studies of the Saccharomyces cerevisiae RNA polymerase II have led to the identification of a holoenzyme form containing a large multiprotein complex associated with the carboxylterminal repeat domain (CTD) of the largest subunit and a subset of general transcription factors (22)(23)(24)44). The CTDassociated complex contains many components, including GAL11, SUG1, and SRB proteins. The association of SRB proteins with the CTD is consistent with genetic evidence regarding function: mutations in the SRB genes were isolated as suppressors of cold sensitivity caused by truncation of the CTD (31, 44). Evidence indicates that this complex is required to mediate transcriptional activation, and it has been designated the mediator (15,22,23).Our studies of genes that contribute to glucose repression have unexpectedly led us to two proteins in the mediator. We recently identified a new cyclin-dependent kinase-cyclin pair encoded by SSN3 and SSN8 (25). Sequence comparison indicates that SSN3 and SSN8 are identical to SRB10 (also known as UME5 and ARE1 [43,52]) and SRB11, respectively, which are new members of the SRB suppressor family encoding proteins in the mediator complex (27). Mutations in SSN3 and SSN8 are members of a set of suppressors of snf1, designated ssn1 through ssn8 (5). These ssn mutations suppress growth defects of a mutant lacking the SNF1 protein kinase, which is required to relieve glucose repression of gene expression (6). The identities between these two pairs of SSN and SRB genes link two genetically defined sets of suppressors and provide an unanticipated functional connection between the SSN family and RNA polymerase II.The functions of two genes in the SSN family have been extensively characterized. SSN1, which is the same as MIG1 (51), and SSN6 have been directly implicated in transcriptional repression. The SSN6 protein, together with TUP1, forms a complex (54) that represses transcription of many genes (38, 47). The SSN6-TUP1 complex is tethered to differently regulat...