No abstract

Single-molecule fluorescence experiments reveal how DEAD-box proteins unfold structured RNAs to promote conformational transitions and refolding to the native functional state.

In embryonic nuclei of DrosophiZa viriZis, 45% of the DNA is satellite, and 50% of the Hl histone is phosphorylated. In polytene salivary gland nuclei, < 1% of the DNA is satellite, and < 10% of the Hl is phosphorylated(1). The decreased Hl 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 Hl histones in adult head cells, which contain the diploid level of satellite DNA, and are predominantly non-dividing. Nearly half of the Hl in these cells is phosphorylated. Thus, the decreased phosphorylation of Hl in polytene cells is correlated with the under-replication of satellite DNA and not with cell replication. The phosphorylated Hl's migrate 4% slower than the unphosphorylated Hl's on SDS-acrylamide gels. The mobility difference may arise because the phosphorylated and unphosphorylated Hl's have different conformations in SDS. This putative conformational difference could be essential to the compaction of satellite DNA into heterochromatin. INTRODUCTIONWe have previously proposed that phosphorylated HI histones compact the