BIOLOGICAL OVERVIEW

Throughout the first 10 nuclear division cycles, occurring over the course of the first three hours of development, the fly embryo remains transcriptionally silent. How is this silence maintained, and how does the transition to zygotic transcription take place? One place to look for an answer is High mobility group protein D (HMG-D).

A phenomenal amount of HMG-D is present in the egg and in the early embryo. It is estimated that there are approximately 10,000 molecules per nucleosome (the multiprotein complex of histones that lends structure to DNA) in earliest of embryos. However, by the cellularization stage [Image] there are only from 2 to 5 molecules of HMG-D per nucleosome, and still later in embryogeneis, less than 0.2 molecules per nucleosome (Ner, 1994).

In general, nuclei which stain strongly for HMG-D are transcriptionally inactive, suggesting that the chromatin generated in the presence of HMG-D is transcriptionally silent. The period of transcriptional silencing ends at the mid-blastula transition, around nuclear cycle 10. The involvement of histone H1 in the midblastula transition in Xenopus (see below: The mid-blastula transition in Xenopus), raises the question of whether H1 could be similarly involved in Drosophila.

The first appearance of H1 takes place during cycles 7 and 8. During cycle 7 the size of the nuclei begins to decrease. By cycles 10-12 a sufficient amount of H1 has accumulated to allow the reorganization of chromatin to a transcriptionally active state. Subsequently, increased zygotic transcription elevates H1 levels even further. This exponential increase, together with the increased number of nuclei, rapidly deplete the relative levels of HMG-D protein.

It has been suggested that the chromatin generated in the presence of HMG-D is transcriptionally silent, and that transcription begins only when H1 levels have reached a particular threshold value and overcome the HMG-D effects, around nuclear cycle 10 (Ner, 1994).

HMG-D is present in later stages of embryogenesis, as is HMG-Z (genetically, a closely linked protein with extensive sequence homology to HMG-D). The combined levels of these proteins during the later stages of embryonic and larval development remain relatively constant and do not exceed the values per nucleosome for H1 (Ner, 1994).

Of course, HMG-D is not the only factor responsible for transcriptional silencing during early development. Maternally supplied Tramtrack protein helps to establish the timing of the onset of zygotic expression of even-skipped and fushi tarazu, thereby preventing premature activation (Read,1992). Initiation of ftz transcription is a gradual process, mediated by titration of TTK protein. When the local concentration of nuclei is experimentally altered, using a mutation that makes the migration of nuclei to the egg cortex uneven, ftz is activated only in nuclei that attain a high local density in the cortex (Pritchard, 1996).

In addition, maternally provided Polycomb- and Trithorax-group transcripts and/or proteins, are present in concentrations that are usually sufficient to last well into embryonic development. For example, maternally synthesized HP1 is underphosphorylated. The appearance of more highly phosphorylated HP1 isoforms at 1.5-2 h of development coincides with the embryonic stage at which cytologically visible heterochromatin appears (HP1 concentrates in heterochromatin). The extent of HP1 phosphorylation is lower in polytene tissue, where heterochromatin is underrepresented (Eissenberg, 1994).

It would seem that the complete story has yet to be told, explaining early gene silencing and later activation at the mid-blastula transition. Equally interesting is the maintenance of gene silencing in pole cells, the stem cells for the fly's gonads. Pole cells are not competent for transcription until gastrulation has begun. HMG-D concentration is high in pole cells, suggesting a role for silencing here as well. It is clear that a major transition takes place in DNA structure at the time of mid-blastula transition. The change in DNA structure involves the replacement of HMG-D with histone H-1, the alleviation of transcriptional repression and the activation of transcripion of specific genes in a rigidly timed manner. This complex transition in such a short time is one of the wonders of development.

GENE STRUCTURE

Two transcripts are found: they have the same 5' end but differ in their 3' ends. The longer transcript (1.3-kb) has an extended 3' untranslated region, suggesting alternative polyadenylation (Wagner, 1992).

cDNA clone length - 955

Bases in 5' UTR - 98

Number of exons - 3

Bases in 3' UTR - 496

PROTEIN STRUCTURE

Amino Acids - 112

Structural Domains

Bovine HMG1 and yeast NHP6 are two proteins with significant similarity to HMG-D (respectively, 36% and 33 % amino acid identity). In addition, a previously identified DNA-binding motif, referred to as the HMG box, is present in the HMG-D protein sequence (Wagner, 1992).

The solution structure of HMG-D shows an extended amino-terminal region and three alpha-helices that fold into a characteristic 'L' shape. The central core region of the molecule is highly stable and maintains an angle of approximately 80 degrees between the axes of helices 2 and 3. The structure determined for the HMG-box motif from HMG-D is essentially identical to the structure determined for the B-domain of mammalian HMG-1. Since these proteins have significantly different sequences these results indicate that the global fold and the mode of interaction with DNA are also likely to be conserved in all eukaryotes (Jones, 1994).

date revised: 11 May 99

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