THE UNI CHROMOSOME OF CHLAMYDOMONAS - HISTONE GENES AND NUCLEOSOME STRUCTURE

The uni linkage group (ULG) of Chlamydomonas reinhardtii contains many genes involved in the basal body-flagellar system, Recent evidence su ggests that the corresponding uni chromosome is located in close proxi mity to the basal body complex. In the course of studies into its mole cular organization, we have found a cluster of four histone genes on t he ULG. The genes are arranged as divergently-transcribed pairs: H3-H4 and H2B-H2A. Genomic sequencing reveals that these genes lack introns and contain characteristic 3' palindromes similar to those of animals , The predicted amino acid sequences are highly conserved across speci es, with greatest similarities to the histone genes of Volvox. Souther n analysis shows that each histone gene is present in 15-20 copies in Chlamydomonas and suggests a dispersed genomic organization. Northern analysis of mitotically-synchronized cells shows that, like the replic ation-dependent histones of higher eukaryotes, Chlamydomonas histone g enes are expressed during S-phase. Using a gene-specific probe on Nort hern blots, we provide evidence that the ULG H4 gene is regulated in t he same manner as other Chlamydomonas histone genes. Finally, micrococ cal nuclease protection experiments show that the uni chromosome itsel f associates with histone proteins and displays a conventional nucleos omal banding pattern.