Molecular characterization of two light-induced, gamete-specific genes from Chlamydomonas reinhardtii that encode hydroxyproline-rich proteins.

Gametic differentiation in Chlamydomonas reinhardtii is a two-step process, which is controlled by the sequential action of the two extrinsic signals, nitrogen starvation and blue light. The gamete-specific genes GAS28 and GAS29 are expressed in the late phase of gametogenesis. Their light-induced expression is restricted to cells that have completed the first, nitrogen starvation-activated, phase of differentiation. A comparison of the two genes revealed striking similarities as well as differences. Their most prominent shared feature is an extended sequence homology of over 90% in their 5'-untranslated regions, suggesting a role in translational regulation. GAS28 and GAS29 both encode hydroxyproline-rich proteins (HRGPs) of very similar sizes that exhibit typical features of volvocalean cell wall constituents. GAS28 shows a high degree of homology with the Volvox pherophorin gene family, suggesting a relationship between these genes.