The Hybrid Histidine KinasedhkBRegulates Spore Germination inDictyostelium discoideum

Abstract

Spore germination is a defined developmental process that marks a critical point in the life cycle of Dictyostelium discoideum. Upon germination the environmental conditions must be conducive to cell growth to ensure survival of emerged amoebae. However, the signal transduction pathways controlling the various aspects of spore germination in large part remain to be elucidated. We have used degenerate PCR to identify dhkB, a two-component histidine kinase, from D. discoideum. DhkB is predicted to be a transmembrane hybrid sensor kinase. The dhkB-null cells develop with normal timing to give what seem to be mature fruiting bodies by 22 to 24 h. However, over the next several hours, the ellipsoidal and encapsulated spores proceed to swell and germinate in situ within the sorus and thus do not respond to the normal inhibitors of germination present within the sorus. The emerged amoebae dehydrate due to the high osmolarity within the sorus, and by 72 h 4% or less of the amoebae remain as spores, while most cells are now nonviable. Precocious germination is suppressed by ectopic activation of or expression of cAMP-dependent protein kinase A. Additionally, at 24 h the intracellular concentration of cAMP of dhkB- spores is 40% that of dhkB+ spores. The results indicate that DHKB regulates spore germination, and a functional DHKB sensor kinase is required for the maintenance of spore dormancy. DHKB probably acts by maintaining an active PKA that in turn is inhibitory to germination.

ForC, a novel type of formin family protein lacking an FH1 domain, is involved in multicellular development inDictyostelium discoideum

“…The time after the onset of starvation is indicated below each picture, and the status at each developmental stage is illustrated above the picture. 330 bp H7 gene fragment was amplified as an internal control (Zinda and Singleton, 1998). consistent with the general sequence of events during which the defects caused by the ∆forC mutation became apparent, and strongly supports our conclusion that forC plays a key role during these multicellular stages.…”

Evolutionary crossroads in developmental biology: Dictyostelium discoideum

“…SDF-1, another secreted peptide, acts on ACG to activate PKA (Anjard and Loomis, 2008), and discadenine acts on the histidine kinase DhkB and ACR to upregulate PKA. Discadenine also acts in parallel to high osmolality to keep spores dormant in the spore head (Anjard and Loomis, 2008;Zinda and Singleton, 1998). This redundancy of regulatory pathways highlights the crucial importance of making and germinating spores only at the correct time and place.…”

Cytokinins induce sporulation inDictyostelium

“…The acgA -strain was a kind gift from Pauline Schaap (van Es et al, 1996). To generate the dhkB -/K strain, KP cells were transformed with the construct used in the original disruption of dhkB (Zinda and Singleton, 1998). After selection with blasticidin and sub-cloning, dhkB disruptants were identified using specific primers.…”

“…Disruption of the gene encoding another histidine kinase, DhkB, reduces the proportion of spores to about one-third when culmination is first completed (Zinda and Singleton, 1998). The number of viable spores in dhkB -strains decreases after 25 hours of development such that it is only 3% of wild-type levels by 72 hours, apparently because the spores germinate while still on top of the stalk.…”

“…The number of viable spores in dhkB -strains decreases after 25 hours of development such that it is only 3% of wild-type levels by 72 hours, apparently because the spores germinate while still on top of the stalk. Zinda and Singleton (Zinda and Singleton, 1998) suggested that dhkB -mutant cells were unable to respond to the germination inhibitor, discadenine, to maintain dormancy. DhkB may also play a role in initiating sporulation as double mutants lacking both DhkA and DhkB are much more impaired in spore formation than either of the single mutants lacking only one of these histidine kinases (Wang et al, 1999).…”