Role of Zeste in transvection at the Bithorax locus of Drosophila

Transvection effects in Drosophila

An unusual feature of the Diptera is that homologous chromosomes are intimately synapsed in somatic cells. At a number of loci in Drosophila, this pairing can significantly influence gene expression. Such influences were first detected within the bithorax complex (BX-C) by E.B. Lewis, who coined the term transvection to describe them. Most cases of transvection involve the action of enhancers in trans. At several loci deletion of the promoter greatly increases this action in trans, suggesting that enhancers are normally tethered in cis by the promoter region. Transvection can also occur by the action of silencers in trans or by the spreading of position effect variegation from rearrangements having heterochromatic breakpoints to paired unrearranged chromosomes. Although not demonstrated, other cases of transvection may involve the production of joint RNAs by trans-splicing. Several cases of transvection require Zeste, a DNA-binding protein that is thought to facilitate homolog interactions by self-aggregation. Genes showing transvection can differ greatly in their response to pairing disruption. In several cases, transvection appears to require intimate synapsis of homologs. However, in at least one case (transvection of the iab-5,6,7 region of the BX-C), transvection is independent of synapsis within and surrounding the interacting gene. The latter example suggests that transvection could well occur in organisms that lack somatic pairing. In support of this, transvection-like phenomena have been described in a number of different organisms, including plants, fungi, and mammals.

Homeosis and the interaction of zeste and white in Drosophila

Transvection effects in Drosophila melanogaster suggest a form of gene modulation that is responsive to the proximity of homologous genes. These effects have been well characterized at bithorax and decapentaplegic, and in the interaction between the zeste and white genes. The mechanistic basis for transvection is not known. As part of a genetic analysis of transvection, a study is being made of a class of mutations defined as modifiers of the eye color resulting from the interaction of zeste and white. This report details the observations that several of these mutations also have homeotic effects.

The Drosophila zeste gene and transvection

The zeste locus of Drosophila melanogaster encodes a DNA-binding protein that can influence transcription. A number of sites recognized by this protein fall within genes associated with transvection, a phenomenon suggesting a form of gene regulation that is responsive to the proximity of a gene to its homologous allele on another chromosome. These recent observations continue a history of studies concerning zeste and transvection which has inspired molecular models linking chromosome structure and positioning to the modulation of gene expression.

Genetic interactions of the suppressor 2 of zeste region genes

A wide variety of gain of function mutations have been induced in the Posterior Sex Comb (Psc)--Aristapedioid (Arp)--Suppressor 2 of zeste (Su(z)2) region of the second chromosome of Drosophila. This region contains at least three apparently related genes, two of which we have been studying. Psc1 has previously been used to identify Psc as a Pc group gene; however, it is a complex mutation with both gain and loss of function character. We report here that the Pc group character of Psc is not due to a gain of function and presumably reflects the function of the wild-type gene. We also provide evidence for a maternal function for Psc, as well as the neighboring Su(z)2 gene. Su(z)2 does not appear to be a Pc group gene as it does not act in a synergistic fashion with other Pc group genes in promoting posteriorly directed transformations. However, we have found that mutations in Su(z)2 do interact in a variety of interesting ways with mutations in Pc group genes.

Studies on transvection at the bithorax complex in Drosophila melanogaster

We have studied the influence of some mutations in the bithorax complex on the observed synapsis dependent phenotype of the genotypes Cbx1Ubx1/+ and bx3 4e/Ubx1. The effect of these mutations is similar to that introduced by disruption of pairing or by the za mutation. Among the bx mutations, we find that bx8 behaves differently from most other bx mutations in its influence on the synapsis dependent phenotype. This observation induced us to map the position of bx8 with respect to other bx mutations; we find that it maps between bx3 4e and bx3. We show how some of the observations reported here can be fitted into a model of activation of the bithorax complex proposed by one of us.

Evidence that two mutations, wDZL and z1, affecting synapsis-dependent genetic behavior of white are transcriptional regulatory mutations

Both the white mutant allele, wDZL, and the zeste mutant allele, z1, reduce white transcript levels in adult head tissues, but have no effect on these levels in other tested adult tissues. wDZL results from insertion of a complex transposon approximately 5.0 kb 5' to white. Juxtaposition of this transposon to white produces a novel transcription unit beginning in the transposon and producing a mature transcript containing both white and transposon sequences. Head-specific expression of this novel transcription unit correlates with the observed repression of wild-type white transcript levels. wDZL and z1 are known to cause synapsis-dependent repression of w+ expression in trans as assessed by eye pigment deposition. Collectively, these results suggest an unexpected and informative correspondence between cis and synapsis-dependent trans regulatory effects. We discuss the implied mechanistic relationship between regulation of transcription and transvection effects in Drosophila.