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 GA S29 are expressed in the late phase of gametogenesis. Their light-induced e xpression is restricted to cells that have completed the first, nitrogen st arvation-activated, phase of differentiation. A comparison of the two genes revealed striking similarities as well as differences. Their most prominen t shared feature is an extended sequence homology of over 90% in their 5'-u ntranslated regions, suggesting a role in translational regulation. GAS28 a nd GAS29 both encode hydroxyproline-rich proteins (HRGPs) of very similar s izes that exhibit typical features of volvocalean cell wall constituents. G AS28 shows a high degree of homology with the Volvox pherophorin gene famil y, suggesting a relationship between these genes.