La Crosse virus nucleocapsid protein controls its own synthesis in mosquito cells by encapsidating its mRNA

Within 24 to 48 h of La Crosse virus infection of mosquito cells, greater than 75% of the S mRNA was found to band in CsCl density gradients at the position of genome or antigenome nucleocapsids. The encapsidation of the S mRNA correlates with the repression of N protein synthesis in vivo, and the encapsidated S mRNA cannot be translated in vitro. Unlike genome and antigenome assembly, S mRNA assembly is a relatively slow process, which is not coupled to its synthesis. Within the encapsidated S mRNA population, three forms could be distinguished, those with intact primers which were or were not also assembled with N protein and those in which the primer and up to 3 template bases had been lost. We suggest that genome replication, but not transcription, is down regulated with time in mosquito cells for reasons that are unclear. The pool of unassembled N protein then increased to the point at which it began to interact with its own mRNA, as this mRNA also contains what is considered to be the assembly site, i.e., the conserved sequences at the 5' ends of all genome and antigenome chains. This lead to the assembly of the entire mRNA, except for the nontemplate primer. Some of the primers were then also assembled with N protein, whereas others were digested to produce truncated mRNAs.