P21X MESSENGER-RNA IS EXPRESSED AS A SINGLY SPLICED PX TRANSCRIPT FROM DEFECTIVE PROVIRUS GENOMES HAVING A PARTIAL DELETION OF THE POL-ENV REGION IN HUMAN T-CELL LEUKEMIA-VIRUS TYPE 1-INFECTED CELLS

In addition to the three typical transcripts such as genomic/gag-pol m RNA, env mRNA and tax/rex mRNA, we previously found the singly spliced pX mRNA, termed p21X mRNA, responsible for producing the p21X protein in human T-cell leukemia virus type 1 (HTLV-1)-infected cells. Our fi nding of the p21X mRNA being constitutively expressed in the fresh per ipheral blood mononuclear cells (PBMCs) from patients with ATL has sug gested that the expression mechanism is quite different from that of t he others. In this paper, the expression mechanism of p21X mRNA was in vestigated by analyzing the organization of the proviral genomes prese nt in the representative HTLV-1-infected cell lines which are positive or negative for the expression of p21X mRNA. Southern and PCR analyse s show that most of the analyzed cell lines contain both one complete and one defective genome each. However, one cell line without the p21X mRNA expression, C91/PL, contains only the complete genome, suggestin g that the complete HTLV-1 has no ability to express p21X mRNA in spit e of having the ability to produce the infectious virus. The defective genomes of the p21X mRNA positive cell lines, MT-2 and H582, have a l arge deletion of the entire pol and parts of the gag and env regions i ncluding the common domain of the second exon of the doubly spliced ta x/rex mRNA, while another defective genome of the p21X mRNA negative c ell line, MT-1, has a deletion within the gag-pol gene. We show that t hese defective genomes have the ability to express their distinct, def ective genomic mRNA, suggesting they are active. The defective genomic mRNAs in MT-2 and H582 cells retain the first splice donor and the se cond splice acceptor sites, suggesting the possibility of synthesizing p21X mRNA by splicing singly with these sites. These findings assume that defective HTLV-1 genomes deleting the second exon region acquire the ability to express p21X mRNA but no ability to express tax/rex mRN A. Such a deletion may explain the difference between the expression m echanisms in the p21X mRNA transcript and those in the other viral tra nscripts.