Characterization of the P25 silk gene and associated insertion elements in Galleria mellonella

Insect silk genes attract attention by their precise territorial and developmental regulations and extremely high expression rates. Our present investigations demonstrated that the P25 silk gene of Galleria mellonella is down-regulated by ecdysteroid hormones. The gene was identified within 5217 nucleotides (nt) of two genomic clones. In contrast to other silk genes, Galleria P25 lacks the canonical TATA box. Transcription is initiated within a region of three nucleotides that lie at the end of a capsite initiator sequence ACAGT and about 90 nt downstream from a CAAT box. A stretch of 32 nt with a core sequence CTTTT was detected in the 5' region of Galleria P25 as well as in the presumptive regulatory regions of all other silk genes that are expressed in the posterior silk gland. However, consensus sequences reported for the regulatory regions of Bombyx silk genes are not obvious in Galleria P25. The coding sequence of this gene included 654 nt, is interrupted by 4 introns, and ends in position +3369; a potential polyadenylation signal starts at +4382. The gene contains 3 copies of a short interspersed nuclear element (SINE), which are located in the upstream region (-833 to -579) and in the first (+542 to +840) and second (+2259 to +2556) introns. The repeat, which was named Gm1, occurs in some other Galleria genes and exhibits homology to Bm1 SINE of the silkworm and to a similar element of a spider. Another insertion of at least 150 nt and with loosely defined borders is present in the 3' untranslated region (UTR) of Galleria P25. It includes a box (+3453 to +3552) of 99 nt that is tentatively called Lep1 because it was disclosed also in some other Lepidoptera. Lep1 seems to represent the core region of insertion elements that occur in the genomes of lepidopteran insects in various species specific and region specific modifications.