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 bel ongs to the HMG1 family, defined by an 80 amino acid DNA binding domain, wh ich includes six highly conserved residues. These amino acids were mutated in Nhp6A and their affects on Nhp6 function were assessed in vivo. Surprisi ngly, most of the changes allowed Nhp6A to function normally in supporting growth at 38 degreesC. However, six mutants had differential effects on in vivo function. Finally, two of the mutant proteins that did not restore Nhp 6A function in vivo were shown to bind and bend DNA in vitro as well as wil d type. Together, these results suggest that Nhp6 interacts with another pr otein(s) to carry out some of its biological functions and that this intera ction might differ at promoters transcribed by RNA polymerase II versus RNA polymerase III. (C) 2001 Academic Press.