Two alternative promoters drive the expression of the cytoplasmic actin A4 gene of Bombyx mori

By screening cDNA and genomic libraries, we have cloned A4, the fourth and last actin gene of Bombyx mori, which encodes a typical cytoskeleton actin and is expressed in all larval tissues. A4 is closely related to A3, another cytoplasmic actin gene of the silkworm, in its encoded amino-acid sequence, and the location as well as the sequence of a single intron. Both A3 and A4 have possibly arisen from the recent duplication of an intron-containing ancestral gene. The two genes display different organization of their 5' untranslated and flanking sequences. In contrast to A3, which harbours a single promoter, A4 exhibits two leader exons transcribed by the use of alternative promoters. A3 and A4 actins differ only by two amino acids at positions known to vary among cytoplasmic actins of other species, and are likely to be functionally equivalent. We speculate that transcriptional constraints are actually the target of a selective pressure that maintains two distinct cytoplasmic actin genes in insects, as well as in other animals.