AVIAN SERUM RESPONSE FACTOR EXPRESSION RESTRICTED PRIMARILY TO MUSCLE-CELL LINEAGES IS REQUIRED FOR ALPHA-ACTIN GENE-TRANSCRIPTION

Serum response factor (SRF) gene expression in avian embryonic muscle lineages plays a central role in activating alpha-actin gene activity. In early stage HH 6 avian embryos, SRF mRNA expression showed strong localization to the neural groove, primitive streak, lateral plate mes oderm, and Hensen's node, while distinct SRF expression was seen later in the neural folds and the somites by HH stage 8. SRF transcripts ap peared in the precardiac splanchnic mesoderm in stage HH 9 embryos and was detected at higher levels in the myocardium, somites, and lateral mesoderm of HH 11 embryos. SRF antibody staining demonstrated signifi cant SRF protein accumulation in the myocardium of the developing hear t and the myotomal portion of somites. During primary myogenesis in cu lture, SRF transcripts and nuclear SRF protein content increased about 40-fold, as primary myoblasts withdrew from the cell cycle, reaching their highest levels prior to the upregulation of the skeletal alpha-a ctin gene. A dominant-negative SRF mutant, SRFpm1, which inhibited DNA binding, but not dimerization of monomeric SRF subunits, blocked tran scriptional activation of a skeletal alpha-actin promoter-luciferase r eporter gene during myogenesis. Transcriptional blockade was reversed by co-transfections of a wild-type SRF expression vector, but was not rescued by the expression of other myogenic factors, such as MyoD and Mef-2C. Thus, SRF displayed an embryonic expression pattern restricted primarily to striated muscle cell lineages, in which increased mass o f nuclear SRF was obligatory for alpha-actin gene transcription. (C) 1 996 Academic Press, Inc.