Figure 4–32 The discovery of position effects on gene expression. The White gene in the fruit fly Drosophila controls eye pigment production and is named after the mutation that first identified it. wild-type flies with a normal White gene (White+) have normal pigment production, which gives them red eyes, but if the White gene is mutated and inactivated, the mutant flies (White–) make no pigment and have white eyes. In flies in which a normal White gene has been moved near a region of heterochromatin, the eyes are mottled, with both red and white patches. The white patches represent cell lineages in which the White gene has been silenced by the effects of the heterochromatin. In contrast, the red patches represent cell lineages in which the White gene is expressed. Early in development, when the heterochromatin is first formed, it spreads into neighboring euchromatin to different extents in different embryonic cells (see Figure 4–31). The presence of large patches of red and white cells reveals that the state of transcriptional activity, as determined by the packaging of this gene into chromatin in those ancestor cells, is inherited by all daughter cells.
Figure 4–33 Some prominent types of covalent amino acid side-chain (B) SERINE PHOSPHORYLATION modifications found on nucleosomal histones. (A) Three different levels of lysine methylation are shown; each can be recognized by a different binding protein and thus each can have a different significance for the cell.
Note that acetylation removes the plus charge on lysine, and that, most importantly, an acetylated lysine cannot be methylated, and vice versa.
(b) Serine phosphorylation adds a negative charge to a histone. modifications of histones not shown here include the monoor dimethylation of an arginine, the phosphorylation of a threonine, the addition of ADP-ribose to a glutamic acid, and the addition of a ubiquityl, sumoyl, or biotin group to a lysine. OPO
As a first step, one can carry out a search for the molecules that are involved.
This has been done by means of genetic screens, in which large numbers of mutants are generated, after which one picks out those that show an abnormality of the process in question. Extensive genetic screens in Drosophila, fungi, and mice have identified more than 100 genes whose products either enhance or suppress the spread of heterochromatin and its stable inheritance—in other words, genes that serve as either enhancers or suppressors of position effect variegation. Many of these genes turn out to code for non-histone chromosomal proteins that interact with histones and are involved in modifying or maintaining chromatin structure. We shall discuss how they work in the sections that follow.
The Core Histones Are Covalently modified at many Different Sites
The amino acid side chains of the four histones in the nucleosome core are subjected to a remarkable variety of covalent modifications, including the acetylation of lysines, the mono-, di-, and trimethylation of lysines, and the phosphorylation of serines (Figure 4–33). A large number of these side-chain modifications occur on the eight relatively unstructured N-terminal “histone tails” that protrude from the nucleosome (Figure 4–34). However, there are also more than 20 specific side-chain modifications on the nucleosome’s globular core.