Cloning and expression of a gene encoding hsc73, the major hsp70-like protein in unstressed rat cells.

The isolation and complete DNA sequence of a rat genomic clone encoding hsc73, the major hsp70-like protein found in growing cells is described. Unlike the heat-inducible genes characterized so far, the hsc73 gene is interrupted by introns, and there are also numerous intronless hsc73 pseudogenes in the rat genome. We show that the levels of hsc73 mRNA are approximately 5-fold higher in rapidly growing tissue-culture cells than in cells whose growth has been arrested by serum starvation. The abundance of hsc73 mRNA is not significantly increased by heat shock of either fed or starved cells. The hsc73 promoter contains putative binding sites for transcription factor Sp1, two CCAAT boxes and, surprisingly, two matches to the consensus heat-shock regulatory element. When fused to the CAT gene and transfected into COS or HeLa cells, the promoter is constitutively active, showing only a small induction by heat shock. Deletion of some constitutive elements makes it more strongly heat-inducible. The gene thus appears to be subject to more than one form of regulation, mediated by different promoter elements that are intermingled.