Polycomb and Trithorax Group Genes in .

Polycomb group (PcG) and Trithorax group (TrxG) genes encode important regulators of development and differentiation in metazoans. These two groups of genes were discovered in by their opposing effects on homeotic gene (Hox) expression. PcG genes collectively behave as genetic repressors of Hox genes, while the TrxG genes are necessary for HOX gene expression or function.

A novel approach for studying histone H1 function in vivo.

In this report, we investigate the mechanisms that regulate Drosophila histone H1 expression and its association with chromatin in vivo. We show that histone H1 is subject to negative autoregulation and exploit this result to examine the effects of mutations of the main phosphorylation site of histone H1.

Transcriptional regulation by trithorax-group proteins.

The trithorax group of genes (trxG) was identified in mutational screens that examined developmental phenotypes and suppression of Polycomb mutant phenotypes. The protein products of these genes are primarily involved in gene activation, although some can also have repressive effects. There is no central function for these proteins.

A histone timer for zygotic genome activation.

Histone H1 variants play key roles in the regulation of higher-order chromatin structure and have been implicated in numerous developmental processes. In this issue of Developmental Cell, Pérez-Montero et al. (2013) present evidence that the Drosophila histone H1 variant dBigH1 prevents premature activation of the zygotic genome during early embryogenesis.

The trithorax group proteins Kismet and ASH1 promote H3K36 dimethylation to counteract Polycomb group repression in Drosophila.

Members of the Polycomb group of repressors and trithorax group of activators maintain heritable states of transcription by modifying nucleosomal histones or remodeling chromatin. Although tremendous progress has been made toward defining the biochemical activities of Polycomb and trithorax group proteins, much remains to be learned about how they interact with each other and the general transcription machinery to maintain on or off states of gene expression. The trithorax group protein Kismet (KIS) is related to the SWI/SNF and CHD families of chromatin remodeling factors.