MOLECULAR ANATOMY OF A SMALL CHROMOSOME IN THE GREEN-ALGA CHLORELLA-VULGARIS

A contig covering the entire region of Chlorella vulgaris chromosome I (980 kb long), consisting of 33 cosmid clones has been constructed. B y cross-hybridization with other chromosomal DNAs, universal structura l elements were detected and localized on the contig. They were compos ed of at least three different elements: short interspersed DNA elemen ts (SINE)-like elements, long interspersed DNA elements (LINE)-like el ements and a putative centromere-like element. At least 36 copies of S INE-like elements were distributed over chromosome I with preferential locations on the right half of the chromosome. DNA fragments containi ng a SINE-like sequence showed a bent or curved DNA nature on polyacry lamide gel electrophoresis. LINE-like elements were clustered at the l eft terminus of chromosome I where they formed a tandem array of six c opies immediately adjacent to the telomeric repeats. A long sequence e lement localized at a unique region of chromosome I also existed in a single copy on each chromosome and contained a sequence related to the reverse transcriptase domain of retrotransposons. This feature was co mpared with the reported centromere-associated elements of higher plan ts. With its comparative simplicity, the organization of Chlorella chr omosome I genomic elements may serve as a prototypic experimental syst em for deciphering the complexity of huge plant chromosomes.