Cloning and pattern of expression of the late histone gene family in the sea urchin species Strongylocentrotus purpuratus and Lytechinus pictus

Abstract

A major problem in developmental biology is the understanding of why different genes are utilized in different cell types. The histone gene family provides an excellent model system for the study of gene action in development because its many subfamilies exhibit both temporal and tissue specificity. As a first step in the understanding of the basic molecular mechanisms of development, I have cloned one subfamily of sea urchin histone genes, the late histones, and studied their pattern of expression during embryogenesis. Transcripts encoding both late H3 and H4 proteins were detected in the unfertilized egg of Lytechinus pictus, suggesting that the late genes were active at some point during oogenesis. Transcription from both the early (another histone subfamily) and late core histones begins at the same point in development, indicating that the pattern of expression is not a switch, as had been thought, but is actually a modulation. Late histone mRNA was observed to accumulate much later in development than early histone mRNA, and changes in the relative rates of transcription appear to be partly responsible for the differential accumulation of these gene products.;To more fully characterize the late histone gene family, I cloned a gene encoding late histone H1 from both Strongylocentrotus purpuratus (H1-gamma) and L. pictus. The predicted primary sequence of both late H1 proteins indicates that they are longer and more highly charged than their early histone counterparts. Unlike the genes encoding the late core histones, the late H1 genes are present only once per haploid genome. Measurement of the accumulation of H1-gamma mRNA by quantitative slot blots demonstrates that late H1 mRNA has a similar pattern of expression as the late core histones, but is present in greater than equimolar quantities after gastrulation.;Comparison of 375 bp of 5{dollar}\sp\prime{dollar} flanking sequences of the two late H1 genes reveals large blocks of identical sequences. Determination of the sequence of the analogous region of the S. purpuratus early H1 gene showed that homology between the flanking sequences of the two subfamilies was limited to consensus sequences present in all H1 genes.