Trypanosomes are protozoan parasites that cause African trypanosomiasis or "sleeping sickness". They reside in the host animal's bloodstream and evade the immune system of the host animal by continually changing the amino acid sequence of their major surface protein. Thus they keep one step ahead of the antibodies directed against them. The objective of this research is to use recombinant DNA techniques to clone and characterize the trypanosome chromosomal gene(s) which code for this variant specific surface antigen (VSSA). These cloned gene sequences will then be used to identify and characterize the molecular mechanisms, at the DNA and RNA level, that generate the amino acid diversity of the VSSA. We have already partially purified polyA ion mRNA that codes for the VSSA in trypanosome Clone #18E2, constructed complementary DNA (cDNA) from this mRNA and cloned that cDNA molecules in E. coli via the plasmid pBR322. We have identified one such recombinant plasmid, pcT602, as containing the coding sequence of VSSA18E2 and determined its complete DNA sequence of 951 base pairs. Random DNA fragments from two immunologically distinct trypanosome clones, Clone #18E2 and #19E1, have been inserted into the bacteriophage lambda and it is proposed to use the VSSA18E2 cDNA to screen for the chromosomal VSSA18E2 gene in both 18E2 DNA and 19E1 DNA. When identified, the chromosomal genes will then be used in a series of DNA/DNA and DNA/RNA filter hybridizations and in heteroduplex analyses to compare the structure of the active VSSA18E2 gene in 18E2 DNA with that of the inactive VSSA18E2 gene in 19E1 DNA. In addition the cloned cDNA will be used as a hybridization probe to study the initial RNA transcripts and their subsequent processing to the final mRNA species coding for the VSSA. And finally the cloned chromosomal genes will be used as substrates for identifying the enzyme or enzyme complex responsible for generating the perpetual amino acid diversity of the VSSA.