A MUTATION IN THE C31-SUBUNIT OF SACCHAROMYCES-CEREVISIAE RNA-POLYMERASE-III AFFECTS TRANSCRIPTION INITIATION

The C31 subunit belongs to a complex of three subunits (C31, C34 and C 82) specific to RNA polymerase (pol) III that have no counterparts in other RNA polymerases. This complex is thought to play a role in trans cription initiation since it interacts with the general initiation fac tor TFIIIB via subunit C34, We have obtained a conditional mutation of pol III by partially deleting the acidic C-terminus of the C31 subuni t. A Saccharomyces cerevisiae strain carrying this truncated C31 subun it is impaired in in vivo transcription of tRNAs and failed to grow at 37 degrees C. This conditional growth phenotype was suppressed by ove rexpression of the gene coding for the largest subunit of pol III (C16 0), suggesting an interaction between C160 and C31. The mutant pol III enzyme transcribed non-specific templates at wild-type rates in vitro , but was impaired in its capacity to transcribe tRNA genes in the pre sence of general initiation factors. Transcription initiation, but not termination or recycling of the enzyme, was affected in the mutant, s uggesting that it could be altered on interaction with initiation fact ors or on the formation of the open complex. Interestingly, the C-term inal deletion was also suppressed by a high gene dosage of the DEDI ge ne encoding a putative helicase.