SSB, encoding a ribosome-associated chaperone, is coordinately regulated with ribosomal protein genes

Genes encoding ribosomal proteins and other components of the translational apparatus are coregulated to efficiently adjust the protein synthetic capa city of the cell. Ssb, a Saccharomyces cerevisiae Hsp70 cytosolic molecular chaperone, is associated with the ribosome-nascent chain complex To determ ine whether this chaperone is coregulated with ribosomal proteins, we studi ed the mRNA regulation of SSB under several environmental conditions, Ssb a nd the ribosomal protein rpL5 mRNAs were up-regulated upon carbon upshift a nd down-regulated upon amino acid limitation, unlike the mRNA of another cy tosolic Hsp70, Ssa, Ribosomal protein and Ssb mRNAs, like many mRNAs, are d own-regulated upon a rapid temperature upshift. The mRNA reduction of sever al ribosomal protein genes and Ssb was delayed by the presence of an allele , EYA3-1, of the gene encoding the heat shock factor (HSF). However, upon a heat shock the EXA3-1 mutation did not significantly alter the reduction i n the mRNA levels of two genes encoding proteins unrelated to the translati onal apparatus. Analysis of gene fusions indicated that the transcribed reg ion, but not the promoter of SSB, is sufficient for this HSF-dependent regu lation. Our studies suggest that Ssb is regulated like a core component of the ribosome and that HSF is required for proper regulation of SSB and ribo somal mRNA after a temperature upshift.