A quantitative electron microscopic analysis of transcription in sea urchin embryos

In an effort to define stage-specific embryonic patterns of sea urchin transcription, we have examined by electron microscopy the distribution of nascent RNP fibrils in dispersed chromatin from nuclei of Strongylocentrotus purpuratus gastrulae. Detailed analysis of individual embryonic nuclei has revealed several new features of nuclear RNA production. Most (82%) of the active chromatin regions observed were represented by only a single fibril. 11% of the active regions contained multiple RNP fibril arrays with an average RNA polymerase density of 1.7 +/- 1.0 polymerases/micron of chromatin and an average contour length of 4.7 +/- 2.8 microns chromatin. An analysis of the lengths of RNP fibrils in single vs. multiple fibril arrays indicates that the differential distribution of RNA polymerases is due to different rates of initiation rather than to different lengths of transcription units (assuming the rate of RNA chain elongation to be constant). We discuss these data in relation to various transcriptional parameters measured by other workers and to EM analyses of other embryonic nuclei. Elucidation of transcriptional patterns in gastrula embryos can provide the basis for further comparative studies of transcription at other stages of sea urchin development in which rates of total genomic transcription vary but the rate at individual loci is as yet unknown.