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

Abstract

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.