| Cytochrome b-559 is a ubiquitous component of PSII. It is comprised of two small polypeptides,αandβsubunit, and a prosthetic group-heme, which is bond by His23 inαsubunit and His23 inβsubunit to form a functional protein. The functional Cyt b-559 is very important in sustaining PSII stability. Based on the previous studies that His binding heme was substituted by other amino acid(s), leading to the deficiency of functional Cyt b-559 and PSII stability, we made mutation of Chlamydomonas reinhardtii that Arg18 and Ser24 upstream and downstream of His23 inαsubunit were replaced with Gly, Glu and Ser, respectively using site-directed mutagenesis. The new results were obtained as follows: All the mutants could be grown photoautotrophically. However, their growth speed was very slow either on HS or TAP medium as compared with the control; Analysis of PSII activity showed that its oxygen evolution and Fv/Fm fluorescent parameter were 50-80% and 40-70% of wild type, respectively. After 10 min of irradiance of saturated light (1000μE·m-2·s-1), the oxygen evolution for all the mutants of Chlamydomonas reinhardtii was declined to zero, and that for wild type was still kept at 35% level; Analyses of the extracted thylakoid membrane proteins using SDS-PAGE and Western-blotting indicated that they were not significant difference from that of the wild type. These data suggested that: the change of one proper amino acid around the heme environment influences the growth speed of mutant cells and their PSII activities although their thylakoid membrane protein components and assembly were not greatly affected; All the mutants were sensitive to strong light irradiance, resulting in that they decreased their ability of protection from light. The conclusion is that Arg and Ser near the heme-bond environment, especially, the alkaline amino acid-Arg is indispensable in maintaining functional Cyt b-559 and PSII oxygen activity, etc.
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