Anomalous electrophoretic mobility of Drosophila phosphorylated H1 histone: is it related to the compaction of satellite DNA into heterochromatin?

In embryonic nuclei of Droeophila virilis , 45% of the DNA is satellite, and = 50% of the H1 hlstone is phosphorylated. In polytene salivary gland nuclei, < 1% of the DNA is satellite, and < 10% of the H1 is phosphorylated (1). The decreased H1 phosphorylation in these non-dividing cells could be correlated either with the under-replication of satellite DNA, or with the absence of mitotic activity. To distinguish between these alternatives, we analyzed the H1 histones in adult head cells, which contain the diploid level of satellite DNA, and are predominantly non-dividing. Nearly half of the H1 in these cells is phosphorylated. Thus, the decreased phosphorylation of H1 in polytene cells is correlated with the under-replication of satellite DNA and not with cell replication. The phosphorylated H1's migrate 4% slower than the unphosphorylated H1's on SDS-acrylamide gels. The mobility difference may arise because the phosphorylated and unphosphorylated H1's have different conformations in SDS. This putative conformational difference could be esse ntial to the compaction of satellite DNA into heterochromatin.