THE PRESPORE-LIKE CELLS OF DICTYOSTELIUM HAVE CEASED TO EXPRESS A PRESPORE GENE - ANALYSIS USING SHORT-LIVED BETA-GALACTOSIDASES AS REPORTERS

In transgenic strains of Dictyostelium discoideum that express P-galac tosidase under the control of a prespore-specific promoter, only early slugs show reporter confined to the prespore zone. As slugs migrate b eta-galactosidase-positive cells accumulate in the prestalk zone; ulti mately, there may be so many that the prestalk-prespore boundary is no longer distinguishable (Harwood, A., Early, A., Jermyn, K. and Willia ms, J. (1991) Differentiation 46, 7-13). It is not clear whether these 'anomalous' reporter-positive cells currently express prespore genes; another possibility is that they are ex-prespore cells that have tran sformed to prestalk and sorted to the prestalk zone (Sternfeld, J. (19 93) Roux Archiv. Dev. Biol. 201, 354-363), while retaining their previ ously produced reporter. To test the activity of the prespore genes in these cells, we have made prespore reporter constructs whose products decay quickly; these are based on constructs used to investigate prot ein turnover in yeast (Bachmair, A., Finley, D. and Varshavsky, A. (19 86) Science 234, 179-186). In strains bearing such constructs, beta-ga lactosidase-positive cells do not appear in the prestalk zone. The app arent deterioration of the prestalk/prespore pattern in older slugs is thus an artefact of reporter stability.