Organization and expression of three genes from the silkmoth early chorion locus

Chorion genes in the silkmoth Bombyx mori are clustered in a small region of one chromosome. Genes are grouped within this locus according to their expression in either early, middle, or late stages of choriogenesis. The entire set of late genes and a large fraction of the middle genes have been cloned and extensively characterized. We report here the first molecular characterization of a set of early chorion genes clustered within a 22.5-kb region. Transcripts homologous to these genes appear in the very first choriogenic follicle but disappear with different kinetics. One of the three early genes, 5H4, is the first example of a chorion gene which is not a member of a multigene family, or divergently paired with another chorion gene. The other two early genes, (ErA.1 and ErB.1) are divergently paired with each other in the manner of the middle and late chorion genes. The common 5′ region between ERA.1 and ErB.1 is significantly shorter and does not contain certain sequence elements shared by the highly conserved 5′ regions of middle and late chorion gene pairs. This evidence taken together with the fact that 5H4 is unpaired indicates that the early genes may utilize a regulatory mechanism somewhat different from the middle and late gene families.