D-mef2: a Drosophila mesoderm-specific MADS box-containing gene with a biphasic expression profile during embryogenesis.

We have identified a mesoderm-specific Drosophila gene, designated D-mef2. The encoded protein contains the MADS- and MEF2-specific domains, which are characteristic of the myocyte-specific enhancer factor 2 (MEF2) family of transcription factors. D-mef2 RNA is first detectable in the presumptive mesoderm at late cellular blastoderm stage and is expressed in all mesoderm after invagination. Following the dorsal migration of the mesodermal layer, D-mef2 expression becomes restricted to the primordia for visceral muscle and the heart. In the second phase, D-mef2 expression is first distinct in heart precursors and then becomes prominent sequentially in visceral and somatic muscles. twi activity is required for D-mef2 expression, while sna function may be needed for the maintenance of D-mef2 expression but not its initiation. D-mef expression is not dependent on the function of tin, and embryos that are deficient for the mesodermal gene DFR1 also show normal initiation of D-mef2 expression at blastoderm. These results suggest that D-mef2 could have a function in early mesoderm differentiation and may be required for subsequent cell fate specifications within the somatic and visceral/heart mesodermal layers.

Negative and positive regulators modulate the activity of a silkmoth chorion gene during choriogenesis.

We have used in vitro linker substitution mutagenesis and P-element-mediated transformation in Drosophila to define cis-regulatory elements of the bidirectional promoter of the Bombyx mori A/B.L12 chorion genes in an in vivo situation. Within an essential 112 bp part of this promoter, mutations in two non-contiguous approximately 35 base-pair regions (A1 and A2) result in significant reduction of expression at late choriogenesis. The reduction occurs in both promoter orientations, indicating the existence of bidirectionally active positive elements. Mutations in A1, but not in A2, also lead to premature activation at early choriogenesis, suggesting the existence of a negative element essential for the finely tuned expression profile during choriogenesis. Normally, this element apparently prevents early expression; it also acts bidirectionally. The highly specific effects of these mutations suggest that several trans-acting regulators of chorion gene transcription have been conserved between silkmoths and fruitflies.

Gene evolution and regulation in the chorion complex of Bombyx mori. Hybridization and sequence analysis of multiple developmentally middle A/B chorion gene pairs.

Twenty-two pairs of chorion genes belonging to the A and B multigene families have been characterized and mapped within two segments of a 320 kb (1 kb = 10(3) bases or base-pairs) chromosomal walk in the domesticated silkmoth Bombyx mori. Eighteen of the gene pairs belong to two groups that are typified by the previously characterized A/B.L12 and A/B.L11 chorion gene pairs, and are defined by two respective types of short (approx. 280 base-pairs) bidirectional promoter sequences. In the chromosome, the L12-like and L11-like pairs are interspersed with each other and with the remaining four gene pairs, which have unrelated promoter sequences. We have sequenced the promoter regions and adjacent small exons of all L12-like and L11-like A and B genes in the walk. The L12-like promoters are highly conserved, whereas L11-like promoters are somewhat more variable. Reconsideration of previous data on RNA accumulation and disappearance during choriogenesis, in the light of the sequences, indicates that L12-like genes are developmentally early-middle, while L11-like genes correspond to two developmental subgroups, middle I and middle II. Sequence comparisons of all these promoters, as well as the previously characterized promoters of the developmentally late HcA and HcB genes, identify short elements of possible regulatory significance. The sequences, as well as extensive cross-hybridization analysis with short probes derived from the reference A/B.L12 gene pair, under carefully controlled conditions of stringency, indicate the occurrence of sequence transfers among A or B genes. These sequence transfers, which could result from gene conversions or unequal crossovers, are less abundant than in the HcA and HcB families, but do result in a patchwork of similarities and differences in the A and B genes. The transfers appear to be least frequent between the moderately divergent A genes that belong to different temporal classes, while the L12-like and L11-like B genes appear to be extensively homogenized in sequence.