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Two tandem genes code for a protein recognized by antibodies to gp24, a small surface protein implicated in cell-cell adhesion of Dictyostelium discoideum. However, these genes do not appear to code for gp24 itself. We have transformed cells with a cloned vector that directs the synthesis of antisense RNA from one of the pair of tandemly repeated genes during the first 4 hr of development. During this period, gp24 protein fails to accumulate and the cells do not become mutually adhesive. Antisense RNA encoded by this vector is unstable and rapidly disappears after synthesis stops. gp24 protein then accumulates and the cells acquire cell-cell adhesion and proceed through development.

The glycoprotein gp24 has been implicated in cell-cell adhesion of Dictyostelium discoideum. We have used a cDNA clone that codes for gp24 to screen cloned genomic fragments. Two closely linked genes (GP24A and GP24B) were recognized that generate mRNAs of about 650 base pairs after excision of a small intron and addition of poly(A). They appear to have arisen by tandem duplication of about 800 base pairs, followed by divergence. These genes are expressed within a few hours of the initiation of development; their mRNAs accumulate to a peak at 12 hr and persist until culmination. Both genes have short guanine-rich sequences (G boxes) upstream that have been shown to be involved in transcriptional regulation of other genes expressed during development of Dictyostelium. Their mRNAs code for proteins that are 85% identical. GP24A and GP24B mRNAs code for proteins with a hydrophobic domain followed by a highly charged carboxyl-terminal domain.