<p style="text-align: justify;">Papaya lines 55-1 and 63-1 were developed using recombinant DNA techniques to resist infection by papaya ringspot virus (PRSV), a major limiting factor in papaya production. These papaya lines were developed by inserting virus-derived sequences that encode the PRSV coat protein (CP). The introduced viral sequences do not result in the formation of any infectious particles, nor does their expression result in any disease pathology. PRSV belongs to the potyvirus group and is an aphid-transmissible RNA virus that commonly infects papaya, causing serious disease and economic loss.</p>

<p style="text-align: justify;">These transgenic papayas exhibit &ldquo;pathogen-derived resistance&rdquo; to infection and subsequent disease caused by PRSV through a process that is related to viral cross-protection. Although the exact mechanism by which the viral protection occurs is unknown, most evidence suggests that expression of viral CP by a plant interferes with one of the first steps in viral replication, uncoating (removal of CP) from the incoming virus (Register &amp; Nelson, 1992). Other modes of action of cross protection have also been suggested (Matthews, 1991).</p>

<p style="text-align: justify;">An antibiotic resistance marker gene (<i>neo</i>) encoding the enzyme neomycin phosphotransferase II (NPTII), which inactivates aminoglycoside antibiotics such as kanamycin and neomycin, was also introduced into the genome of these varieties. This gene was derived from a bacterial transposon (Tn5 transposable element from <i>Escherichia coli</i>) and was included as a selectable marker to identify transformed plants during tissue culture regeneration and multiplication. The expression of the neo gene in these plants has no agronomic significance and the safety of the NPTII enzyme as a food additive was evaluated by the United States Food and Drug Administration in 1994 (US FDA, 1994).</p>

<p style="text-align: justify;">Line 55-1 expresses a reporter gene (<i>uidA</i>) encoding the enzyme beta-D-glucuronidase (GUS) from <i>E. coli</i>. The expression of GUS activity was used during the development phase to identify transgenic plants containing the introduced gene sequences.</p>
