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The 140‐nucleotide spliced leader (SL) RNA, involved in mRNA maturation in the African trypanosomes and in other kinetoplastida, is encoded by a tandem array of spliced leader genes. We show that the 1.4‐kb SL gene repeat unit in Trypanosoma gambiense is organized in tandem arrays confined to two large (minimum size 350‐450 kb) restriction fragments. SL genes in both arrays are interrupted by a total of eight conserved insertion elements. Cleavage of genomic DNA at restriction sites present within the insertion element but not in the SL gene repeat, releases variable numbers of SL genes from the tandem array. Since the insertion element contains a terminal poly(A) track of 36 bases and because a 49‐bp duplication of target DNA has occurred at the integration site, we conclude that it is a retroposon. This retropson is uniquely associated with the SL gene clusters. These retroposons presumably originated from a single insertion event after which their copy number increased, possibly through unequal sister chromatid exchange.

We have constructed a panel of Abelson murine leukemia virus-transformed pre-B cells to study the organization of the mouse VH gene families. Based on the analyses of VH gene deletions on 51 chromosomes with VH gene rearrangements, we have inferred a map order of the Igh locus that holds for both the Igha and Ighb haplotypes. We show that members of each VH gene family are generally clustered, although three family clusters (VHS107, VH36-60, VGAM3.8) are dispersed in two or three subregions of the locus. Members of two VH gene families, VHQ52 and VH7183, are extensively interspersed and map within the same subregion. An examination of the distribution of VH group members (VH II, I, and III) within the locus suggests that two major duplications may, in part, explain the dispersed pattern of VH family clusters. The relationship of VH organization and functional expression is discussed in terms of position-dependent and complexity-driven models.

Infection with the African trypanosomes gives rise to relapsing waves of parasitemia in the host. A predominant population of trypanosomes is present in each wave, and such predominant populations are usually serologically distinct from each other. Trypanosomes are covered by an extramembranous, highly antigenic, variant-specific glycoprotein coat that is 15 nm thick. The primary structure of a large portion of the glycoprotein molecule is different in the predominant trypanosome populations of each parasitemic wave. Analysis of the secondary structure potential of five full-length and five partial amino acid sequences of variant-specific glycoproteins from members of the Trypanosoma brucei complex has been carried out. The potentials for alpha-helix, beta-turns, and beta-strand structure have been calculated. A high degree of alpha-helical structure potential is present in all the full-length or partial sequences examined. There is conservation of secondary structure potential in the COOH-terminal 100 amino acids, where both partial and complete conservation of primary amino acid sequence exists. The NH2-terminal regions are rich in alpha-helix potential. However, over large stretches of the middle of the VSG molecules there is wide diversity of secondary structure potential. This suggests that tertiary folding structures may also be different in this region. If these predictions are true, different regions of the variant-specific glycoprotein could be exposed to the solvent in different variant-specific trypanosome serotypes. The implication is that antigenic variation is mediated by a polygene family of glycoproteins containing highly polymorphic regions. These could fold differently and expose different surface regions of the protein to the solvent. This device might reduce immune crossreactivity among members of the variant-specific glycoprotein family.