Detailed Structure of Pneumocystis carinii Chromosome Ends

Pnewnocystis carinii has three gene families, MSG, MSR and PRTl, the members of which tend to be grouped together in clusters located at chromosome ends (reviewed in reference 6). Each of these gene families encodes a family of surface proteins, which are known as Major Surface Glycoprotein (MSG), MSG-Related (MSR). and Protease (PRTl). Determining the size and composition of these gene arrays is important for understanding their evolution and function. A question of particular interest is the possibility of co-expression of a specific MSG gene and a specific PRT1. because MSG proteins may be processed by proteases e n d e d by PRTl genes [3. 7.1. Prior to the work described here. segments of the genome that contain members of these gene families had been characterird. but an entire cluster had not yet been isolated. As part of the Pneumocystis Genome Project. the Sanger Center (http://www.sanger.ac.uk) determined the sequence of gene arrays contained within two cosmids, 3G5 and 1B2. Both cloned gene clusters contained members of all three gene families, but the order of the genes in the two clones was different. In addition to full size MSG and MSR genes, both gene clusters contained what appear to be fragments of MSG or MSR genes. The library contained additional cosmids that shared with 1B2. and with each other. a core region of about 15 kilobases at the telomere-distal end of the sequence. Surprisingly, two of these 1B2related cosmids mapped to one chromosome, and another mapped to a different chromosome. These linkage data show that the same gene array can be at the ends of two different chromosomes.