A 29-Nucleotide DNA Segment Containing an Evolutionarily Conserved Motif is Required in cis for Cell-Type-Restricted Repression of the Chicken α-Smooth Muscle Actin Gene Core Promoter

Abstract

A series of 5′ deletion mutations of the upstream flanking sequences of the chicken α-smooth muscle (aortic) actin gene was prepared and inserted into the chloramphenicol acetyltransferase expression vector pSV0CAT. Deletion recombinants were transfected into fibroblasts, which actively express the α-smooth muscle actin gene, and primary myoblast cultures, which accumulate much lower quantities of α-smooth muscle actin mRNAs. The first 122 nucleotides of 5′-flanking DNA were found to contain a "core" promoter capable of accurately directing high levels of transcription in both fibroblasts and myotubes. The activity of this core promoter is modulated in fibroblasts by a "governor" element(s) located at least in part between nucleotides -257 and -123. This region contains sequences potentially conserved between mammalian and avian α-smooth muscle actin genes as well as one of a pair of 16-base-pair inverted CCAAT box-associated repeats which are conserved among all chordate muscle actin genes examined to date. A smaller DNA segment (-151 to -123) containing these upstream CCAAT box-associated repeats was sufficient to suppress expression of the core promoter in muscle cultures, suggesting that the upstream CCAAT box-associated repeats play a negative role in the α-smooth muscle actin gene promoter.