CHROMOSOMAL ARCHITECTURE IN GIANT PREMEIOTIC NUCLEI OF THE GREEN-ALGAACETABULARIA

Giant primary nuclei of the unicellular green alga Acetabularia contai n 40 small lampbrush chromosomes which have proved difficult to visual ize in the light microscope in vivo by conventional fluorescent DNA st aining techniques. We report here that immunofluorescence staining wit h the monoclonal anti-phosphoepitope antibody MPM 2 is the method of c hoice to study the architecture of whole chromosomes within primary nu clei fixed in situ or after hand isolation. Using confocal laser scann ing microscopy, we have been able to produce images of Acetabularia la mpbrush chromosomes of hitherto unsurpassed structural detail. Particu larly striking is the visualization of abundant loops extending from t he periaxial chromosome core region for variable distances into the nu cleoplasm. Staining of the loops can be abolished by pretreatment of i solated nuclei with alkaline phosphatase, whereas the chromosomal core remains unaffected. At meiosis, when transcription ceases and the ext ended chromosomes condense, MPM2 staining is lost indicating that main tenance of the loops depends on the phosphorylation status of the MPM 2 antigen. Anti-histone antibodies, on the other hand, exclusively sta in the chromosomal core in the extended lampbrush configuration as wel l as in the condensed metaphase configuration. This indicates that MPM 2 and anti-histone antibodies recognize different molecular component s on the chromosomes. We postulate that the loops stained by MPM 2 rep resent actively transcribing regions of the chromosomes and that the M PM 2 antigen could be a molecular component involved either in the str uctural maintenance of the loops or in a process associated with trans cription.