Multiple, Closely Spaced Alternative 5′ Exons in the DmCKIIβ Gene of Drosophila melanogaster

Drosophila melanogaster casein kinase II (CKII) is composed of catalytic α and regulatory β subunits. Using the two-hybrid system, we have isolated a number of cDNAs that are related to a previously published cDNA encoding the β subunit, but exhibit divergent 5′ sequences. To determine the source of this sequence variation, we have isolated the gene encoding the β subunit of CKII. The β gene contains five exons encompassing the complete open reading frame, as well as five alternative exons in the 5′ untranslated region (UTR). Only one 5′ UTR exon is contained in each cDNA, implying five distinct classes of transcript. In addition, the β gene contains at least two poly(A) addition signals which generate additional complexity at the 3′ end. The complex pattern of transcription may serve a role in the spatial and/or temporal expression of the β subunit since, with one exception, all transcripts encode the full-length β polypeptide. Phylogenetic comparison of the β genes of Drosophila, C. elegans, and mammals reveals three invariant introns as well as evidence of recent intron gain/loss.