Differential expression of the members of the discoidin I multigene family during growth and development of Dictyostelium discoideum.

Discoidin I and II are lectins synthesized during the aggregation of Dictyostelium discoideum amoebae which may play a role in cellular cohesion. Discoidin I was thought to consist of two major polypeptides, but we show that there are three. The N-terminal amino acid sequence of the polypeptides has been predicted by determining part of the nucleotide sequence of their respective mRNAs. We obtained the nucleotide sequences by reverse transcription of the mRNAs, using as primers, fragments derived from the coding region of two cloned discoidin I sequences, and utilizing cross hybridization to the various mRNA species and differences in the length of their 5' noncoding regions to isolate fragments for DNA sequencing. We used primer extension to measure the relative concentration of the three major discoidin I mRNA sequences. We show that during development changes in the abundance of all three mRNA sequences occur coordinately. In cells growing in nutrient broth, however, only two of the three major discoidin I mRNA sequences accumulate, and if such cells are grown to a very high density, both sequences disappear. These results indicate that the coordination of discoidin I gene expression is not obligatory and that the members of this multigene family may differ in the mode of their induction during normal development.