Histone H1 proteins in Chlamydomonas reinhardtii

Abstract

The Hl proteins of the unicellular green alga Chlamydomonas reinhardtii were extracted from isolated nuclei, fractionated by two-dimensional electrophoresis, and analyzed by peptide mapping and N-terminal sequencing. C. reinhardtli histones were compared to histones collected from pea leaf nuclei. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) of 5% perchloric acid (PCA) extracts of isolated C. reinhardtii nuclei revealed two Hl proteins (Hia and Hlb) along with an H2B protein which was partially extracted by the acid. Two-dimensional gel analysis did not reveal heterogeneity of either algal Hl protein, while two-dimensional gels of 5% PCA extracts of pea leaf nuclei separated the three Hl bands seen in SDS-PAGE, into five protein spots. Two-dimensional gels did detect differences in the hydrophobic amino acid content of the two C. reinhardtii Hls as evidenced by changing mobilities of Hla and Hlb relative to each other when Triton was added to the acid-urea gels. Digestion of the algal Hl proteins with V8 protease revealed two distinctly different peptide maps. C reinhardtil Hl peptide maps were not similar to pea Hl peptide maps, but peptide maps of algal and pea H2B did show some peptides in common. Seventeen amino acid residues were obtained from C. reinhardtii Hla amino terminal sequencing, while the Hlb N-terminus was blocked. A search of protein databases revealed no sequence homology of the Hla N-terminus with any known proteins. Changes in HI levels between gametes and vegetative cell types were studied by inducing gametogenesis in C. reinhardtli cultures and analyzing Hlb/Hla ratios in gametes relative to nongamete control cells. This analysis suggested that the ratio decreases in gametes.