Differentiation of BC3H1 myoblasts to myocytes is accompanied by a 67% drop in the rate of rpL32 gene transcription. Addition of high concentrations of serum to resting myocyte populations stimulates cell growth and subsequent dedifferentiation to proliferating myoblasts with a return to the normal rate of rpL32 gene transcription. During these growth rate changes the binding activities of previously identified factors (beta, gamma, delta) which interact with the rpL32 gene promoter were examined by mobility shift assays. Binding of the beta factor (an Ets related protein) to an oligonucleotide containing the beta element was reduced significantly in myocyte nuclear extracts, but subsequent dedifferentiation increased binding within 30 min in either the presence or absence of the cycloheximide. Binding of the gamma and delta factors to their respective elements changed only slightly during these processes. Dephosphorylation of either myoblast or myocyte extracts resulted in increased binding of the beta factor suggesting that binding activity of the beta factor is modulated by phosphorylation during the changes in BC3H1 myoblasts growth rate. In addition, mobility shift assays with recombinant GABP alpha and beta proteins and their specific antibodies revealed that GABP proteins bind to the rpL32 gene promoter in a sequence dependent manner, and that similar proteins are present in BC3H1 myoblast/myocyte extracts. These results support the premise that the GABP heterodimer is the rpL32 beta factor. Furthermore, during BC3H1 myoblast differentiation and dedifferentiation neither the levels of the GABP alpha and beta proteins nor their respective mRNAs change. These results suggest that GABP is a constitutively expressed protein and is involved in regulating rpL32 gene by post-transcriptional modifications.