Overexpression of the csA cell adhesion molecule under its own cAMP-regulated promoter impairs morphogenesis in Dictyostelium.

The contact site A (csA) glycoprotein is a strictly developmentally regulated plasma membrane component responsible for the EDTA-stable (Ca(2+)-independent) form of intercellular adhesion in Dictyostelium discoideum. Using inverse polymerase chain reaction and a terminator vector we have isolated a 1.6 kb genomic fragment carrying a 1.1 kb upstream region of the csA gene. This fragment had promoter activity in D. discoideum cells, giving rise to a 3'-truncated csA RNA that was regulated like the mRNA of the endogenous gene. Cyclic AMP pulses strongly enhanced transcription from the cloned csA promoter. These findings provide evidence that the cloned region of the csA gene comprises the complete promoter. It contains a G/C-rich octamer motif similar to other cAMP-regulated D. discoideum promoters. When the csA protein was strongly overexpressed under the developmental control of the csA promoter, morphogenesis was substantially altered. Aggregation was delayed, and secondary centres were formed along aggregation streams that led to fragmentation of the aggregates and multiple slug formation. At high cell density a substantial portion of aggregated cells was left behind on the substratum when slugs and fruiting bodies were built. The transformation vector was also employed to rescue a csA-negative mutant, HG1287, from its cell adhesion defect.