The second messenger cyclic AMP (cAMP) is very likely involved in phototactic as well as gravitactic behavior of the unicellular flagellate Euglena gracilis. A slight but significant increase in cAMP was observed when cells encountered sub-threshold acceleration (0.16 × g) force after microgravity µg. No differences in cAMP levels were found between cells on a clinostat and 1x-controls. This observation is consistent with the ones of earlier studies. Illumination of cells resulted in a significant increase in cellular cAMP levels. After RNAi-mediated knockdown or CRISPR-Cas9 knockout of the photoactivated adenylyl cyclases PACα and/or PACβ in the photoreceptor, light-induced changes in cAMP levels were no longer observed. In parallel, phototactic behavior was abolished, supporting the essential role of photoactivated adenylyl cyclases in phototaxis. Cells spin around their length axis during locomotion (1–2 Hz). In order to generate a signal in the light direction, the cells should be capable of synthesizing and degrading cAMP within 0.5–1 s. The rapid fixation of cells upon transition from dark to light or light to dark revealed that detectable changes in cAMP-levels (increase or decrease) occur within a 100–200 ms time window, which is sufficiently fast to account for the proposed theoretical kinetics of cAMP oscillations.
Richter et al. (Tue,) studied this question.