No abstract

No abstract

No abstract

Mapping and complementation analysis with 17 phototaxis mutations has established 11 complementation groups phoA-phoK distributed over six linkage groups. Statistical calculations from the complementation data yielded 17 as the maximum likelihood estimate of the number of pho genes assuming all loci are equally mutable. Most of the phototaxis mutants were found to exhibit bimodal phototaxis and all were found to be impaired in positive thermotaxis supporting convergence of the photosensory and thermosensory pathways. The thermotaxis mutant HPF228 was unaltered in phototaxis suggesting that the mutation in this strain affects a gene product whose site of action is before the convergence of the two pathways. Other phenotypes such as multiple tip formation by aggregates, stumpy fruiting bodies with short or absent stalks and short migration were associated with some pho alleles suggesting multiple biological roles for some gene products important in phototransduction.

We recently reported the first molecular genetic evidence that Dictyostelium Ca(2+) responses to chemoattractants include a contribution from the endoplasmic reticulum (ER) - responses are enhanced in mutants lacking calreticulin or calnexin, two major Ca(2+)-binding proteins in the ER, even though the influx of Ca(2+) into the mutants is reduced. Compared with wild-type cells, the ER in the mutants contributes at least 30-70 nM additional Ca(2+) to the responses. Here we report that this additional ER contribution to the cytosolic Ca(2+) signal depends upon extracellular Ca(2+)- it does not occur in the absence of extracellular Ca(2+), increases to a maximum as the extracellular Ca(2+) levels rise to 10 microM and then remains constant at extracellular Ca(2+) concentrations up to at least 250 microM. These results suggest that Ca(2+) influx causes the intracellular release, in the simplest scenario by a mechanism involving Ca(2+)-induced Ca(2+) release from the ER. By way of contrast, we show that Ca(2+) responses to mechanical stimulation are reduced, but still occur in the absence of extracellular Ca(2+). Unlike the responses to chemoattractants, mechanoresponses thus include contributions from the ER that are independent of extracellular Ca(2+).