Gene dosage affects the expression of the duplicated NHP6 genes of Saccharomyces cerevisiae

Nhp6Ap and Nhp6Bp, which are 87% identical in sequence, are moderately abundant, chromosome-associated proteins from Saccharomyces cerevisiae. In wild type cells Nhp6Ap is present at three times the level of Nhp6Bp. The effects of altering NHP6A or NHP6B gene number on the expression of its partner has been examined using Northern blots and reporter genes. Deletion of NHP6A led to a three-fold increase in NHP6B synthesis while an extra copy of NHP6A reduced NHP6B expression two-fold. Changes in the NHP6B gene copy number caused more moderate changes in NHP6A synthesis. The regulation of one NHP6 gene by the other uses a mechanism that detects the level of Nhp6 protein (or RNA) rather than gene number, since overexpression of Nhp6B protein from a single gene led to a dramatic decrease in NHP6A synthesis. Deletion analysis showed that the regulatory element involved in gene dosage compensation maps to a 190 bp segment in the NHP6B promoter. The simplest model, that each Nhp6 protein can act as a transcriptional repressor at the other NHP6 gene, is not true since purified Nhp6A protein does not bind specifically to the NHP6B promoter region. Instead, Nhp6p appears to interact with or through another protein in regulating transcription from the NHP6 genes.

Mutations in the yeast Nhp6 protein can differentially affect its in vivo functions

Nhp6A and Nhp6B from S. cerevisiae are required for viability at 38 degrees C because they are involved in transcription of SNR6 by RNA polymerase III. Nhp6A also represses transcription of NHP6B by RNA polymerase II. Nhp6 belongs to the HMG1 family, defined by an 80 amino acid DNA binding domain, which includes six highly conserved residues. These amino acids were mutated in Nhp6A and their affects on Nhp6 function were assessed in vivo. Surprisingly, most of the changes allowed Nhp6A to function normally in supporting growth at 38 degrees C. However, six mutants had differential effects on in vivo function. Finally, two of the mutant proteins that did not restore Nhp6A function in vivo were shown to bind and bend DNA in vitro as well as wild type. Together, these results suggest that Nhp6 interacts with another protein(s) to carry out some of its biological functions and that this interaction might differ at promoters transcribed by RNA polymerase II versus RNA polymerase III.

Nhp6, an HMG1 protein, functions in SNR6 transcription by RNA polymerase III in S. cerevisiae

Nhp6A and Nhp6B are HMG1-like proteins required for the growth of S. cerevisiae at elevated temperatures. We show that the conditional lethality of an nhp6 strain results from defective transcription of SNR6 (U6 snRNA) by RNA polymerase III. Overexpression of U6 snRNA or Brf1, a limiting component of TFIIIB, and an activating mutation (PCF1-1) in TFIIIC were each found to suppress the nhp6 growth defect. Additionally, U6 snRNA levels, which are reduced over 10-fold in nhp6 cells at 37 degrees C, were restored by Brf1 overexpression and by PCF1-1. Nhp6A protein specifically enhanced TFIIIC-dependent, but not TATA box-dependent, SNR6 transcription in vitro by facilitating TFIIIC binding to the SNR6 promoter. Thus, Nhp6 has a direct role in transcription complex assembly at SNR6.