P25 gene regulation in Bombyx mori silk gland: two promoter-binding factors have distinct tissue and developmental specificities.

Abstract

The gene encoding the silk protein P25 is expressed in the posterior silk gland of Bombyx mori with strict territorial and developmental specificities. The cis-acting regulatory elements previously located within the 441-bp 5' proximal sequence of the gene were examined for protein-binding capacities. We identified two factors, BMFA and SGFB, that lead to prominent band shifts and the target sites for which are included in a region homologous to the fibroin gene enhancer sequence. Analysis of the tissue-specific incidence of both factors showed that BMFA is ubiquitous, whereas SGFB is restricted to the silk gland cells. However, SGFB was found in both posterior and middle silk gland cells and therefore likely directs organ-specific, but not territory-specific, expression. Developmental studies throughout the fourth larval molt, at which the P25 gene status changes from derepressed to repressed, revealed that BMFA is reversibly modified at the transition from intermolt to molt. Indeed, the preexisting BMFA is replaced by a structurally related factor, BMFA', during the 2 h following head capsule apolysis. The exact temporal coincidence of this conversion with the onset of gene repression suggests that BMFA' is involved in transcription inactivation and likely results from a transduction process initiated by the hormonal change at molting.

Differential Binding of the Bombyx Silk Gland-Specific Factor SGFB to Its Target DNA Sequence Drives Posterior-Cell-Restricted Expression

“…This correlates with the footprint on the target DNA in PSG chromatin, strongly suggesting that the factor SGFB, a silk gland-specific protein, acts as a transactivator. In vitro characterization by gel retardation assay, proteolytic clipping, and SGFB-DNA cross-linking did not reveal structural differences between the SGFBs in PSG and MSG cells (5). However, our results clearly show that the silk gland factor stably binds to its target sequence in P25-active cells and not in inactive MSG cells.…”

“…The mutations in the Ub2a, Ub2b, BMFA, and SGFB elements which abolish the binding of their cognate factors (5,24) were created by ligating appropriate fragments resulting from 5Ј and 3Ј BAL 31 digestions of P25 DNA. The wild-type sequences were replaced by the same exact number of nucleotides taken from the plasmid polylinker DNA.…”

“…To identify putative regulatory elements, in vitro DNA-protein interactions between silk gland nuclear factors and synthetic DNA probes were explored, and two possible regulators with different properties were discovered (5). The first, BMFA, is a ubiquitous protein thought to be involved in the repression of silk gland-expressed genes at molting, including those encoding silk proteins.…”

A Juvenile Hormone Transcription Factor Bmdimm-Fibroin H Chain Pathway Is Involved in the Synthesis of Silk Protein in Silkworm, Bombyx mori

“…These results demonstrated that Bmdimm interacted with Bmsage in nuclei. It has been found that an SGF1-forkhead complex acts as a crucial transcription activator that binds to proximal upstream elements of both the fibroin and p25 genes in PSG cells (55). Our previous study demonstrated that Bmsage is involved in regulation of the fib-H gene via interaction with SGF1 (15).…”

Differential Gene Expression in Insects: Transcriptional Control

“…This sequence interacts with a GATA-like factor in conjunction with the activation of late chorion gene expression in B. mori follicle cells (44,45). An example of temporal and tissue region specificity is provided by work on the expression of the P25 gene in the PSG of B. mori (46,73). This tissue region has approximately 500 cells, and expression of the P25 gene is restricted to PSG at a specific time in development.…”