The Drosophila retina has an autonomous peripheral circadian clock in which the expression of the gene encoding heme oxygenase (HO) is under circadian control with the ho mRNA peaking at the beginning of the day and in the middle of the night. The function of HO in the retina is unknown, but we observed that it regulates the circadian clock and protects photoreceptors against DNA damage. The decline in HO level increases and decreases the expression of the canonical clock genes period (per) and Clock (Clk), respectively. The opposite result was observed after increasing HO expression. Among three products of HO activity-carbon monoxide (CO), ferrous ions, and biliverdin-the latter has no effect on per and Clk expressions, but CO exerts the same effect as the increase of ho expression. This suggests that HO action on the clock is mediated by CO, which may affect Clk expression during the day and the level of per expression. While ho expression is not stimulated by nitric oxide (NO), NO has the same effect on the clock as HO, increasing Clk expression and decreasing the expression of per.
Keywords: Carbon monoxide; Circadian clock; Drosophila; Heme oxygenase; Nitric oxide; Retina.