The highly reduced genome of an enslaved algal nucleus.

Chromophyte algae differ fundamentally from plants in possessing chloroplasts that contain chlorophyll c and that have a more complex bounding-membrane topology. Although chromophytes are known to be evolutionary chimaeras of a red alga and a non-photosynthetic host, which gave rise to their exceptional membrane complexity, their cell biology is poorly understood. Cryptomonads are the only chromophytes that still retain the enslaved red algal nucleus as a minute nucleomorph. Here we report complete sequences for all three nucleomorph chromosomes from the cryptomonad Guillardia theta. This tiny 551-kilobase eukaryotic genome is the most gene-dense known, with only 17 diminutive spliceosomal introns and 44 overlapping genes. Marked evolutionary compaction hundreds of millions of years ago eliminated nearly all the nucleomorph genes for metabolic functions, but left 30 for chloroplast-located proteins. To allow expression of these proteins, nucleomorphs retain hundreds of genetic-housekeeping genes. Nucleomorph DNA replication and periplastid protein synthesis require the import of many nuclear gene products across endoplasmic reticulum and periplastid membranes. The chromosomes have centromeres, but possibly only one loop domain, offering a means for studying eukaryotic chromosome replication, segregation and evolution.

HLA-DP epitope typing using monoclonal antibodies.

We have made a panel of murine anti-DP monoclonal antibodies for serological typing of HLA-DP polymorphisms; they can be used in microcytotoxicity (for 7 epitopes) and binding assays (for 8 epitopes). The antibodies detect polymorphic differences in both alpha and beta chains. As immunogens we sometimes used B-lymphoblastoid lines or purified DP molecules but mostly used mouse fibroblast transfectants expressing DP molecules. The DP beta genes were made from a cloned DPB1*0201 gene by replacing its major area of polymorphism with matching stretches of DNA amplified from other alleles; cloned DPA1*01 and DPA1*02 genes were used for transfection along with the beta chain genes. The monoclonal antibodies showed reaction patterns that correlated with the presence of particular amino-acid sequence motifs; thus none of the antibodies is allele-specific. They bind instead to epitopes which are found on a number of different HLA-DP types. We have constructed frequency tables so that the epitope (motif) data can be interpreted as the most likely genotype in each case. The basic assumption to justify this work is that HLA-DP matching or mismatching will likely influence transplant outcome, particularly in bone marrow transplantation. The present challenge is to define permissive and nonpermissive combinations of HLA-DP; it may be that matching for epitopes, rather than for full alleles, will help to resolve this issue.