Transmission of replication-defective Sindbis helper vectors encoding capsid and envelope proteins.

A green fluorescent protein (gfp)-encoding derivative of the replication-defective Sindbis helper vector DHBB was constructed in order to study the rate of packaging of the helper vector. The results show that despite lacking a 'packaging sequence', this vector is co-packaged about 1/300th the rate of packaging of single RNAs containing a packaging sequence. This helps explain the frequent observation of recombinant, replication-competent viruses when using first generation Sindbis packaging systems. Because of their sensitivity, gfp-encoding reporters like the one described here will be useful for measuring transfer of helper RNAs in improved alpha virus packaging systems.