Activation of sea urchin actin genes during embryogenesis. Measurement of transcript accumulation from five different genes in Strongylocentrotus purpuratus. The number of molecules of mRNA transcribed from each of five different actin genes are reported for developing embryos of the sea urchin Strongylocentrotus purpuratus. Transcripts of the cytoskeletal actin genes CyI, CyIIa, CyIIb and CyIIIa, and of the muscle actin gene M, were measured in unfertilized egg and embryo RNAs of cleavage, blastula, gastrula and pluteus stages. The measurements were obtained by probe excess titrations of these RNAs, using a set of single-stranded RNA probes each identifying the mRNA transcripts of a specific actin gene. These mRNAs can be identified by their distinct 3'' non-translated trailer sequences. We confirm prior observations that the prevalence of actin mRNA in the unfertilized egg is low. Cytoskeletal actin genes CyI and CyIIIa each contribute 1 X 10(3) to 2 X 10(3) maternal mRNA molecules, and CyIIb contributes less than 2 X 10(2) mRNA molecules, while no detectable maternal mRNAs derive from cytoskeletal actin gene CyIIa or the muscle actin gene M. During certain periods of development, transcripts derived from the individual cytoskeletal actin genes accumulate rapidly, with kinetics specific to each mRNA. Transcripts of the muscle actin gene are absent until after gastrulation, when the initial muscle progenitor cells are formed. At late stages of development, each of the five genes studied is represented by 10(4) to 10(5) mRNA molecules per embryo. The present measurements permit calculation of the levels of each actin mRNA species in the particular cell types in which each gene functions in the fully differentiated embryo.