**10. Concluding remarks**

small RNAs from the nucleus to cytoplasm. Downregulation of Ran by the expression of *bmnpv-miR-1* in viral infection triggers the reduction of the host small RNA population and increasing of the viral load in infected *B. mori* larvae. In this way BmNPV counteracts the small

It is known that some viruses are capable of persisting in their hosts without causing disease. This can be accomplished by producing either a latent or a persistent infection. The main difference between both is that during latent infection the virus is not replicating and keeping a minimal gene expression while in persistent infections all the genes are expressing, at low levels, without causing any symptom. Herpesviruses can establish latent infections in specific cell types [73]. This state is characterized by a unique transcriptional program that involves the expression of latency-associated transcripts (LATs) as the only viral products synthesized in large quantities. The virus is maintained as an independent quiescent genetic material within the host cell nucleus. An alternative mechanism is observed in measles virus by which the virus remains at low levels with the production of viral proteins. This is usually referred to as

The White Spot Syndrome Virus (WSSV) is a non-occluded virus pathogenic to shrimp, phylogenetically related to baculoviruses. It was found at very low levels in asymptomatic shrimps. The virus may reside within cells in a quiescent state as in a latent infection or causing a persistent infection [75]. Similarly, a nudivirus was found infecting persistently the cell line

Baculoviruses are highly lytic, causing a lethal disease in infected larvae. Epizootics caused by these viruses can reduce dramatically their host population [76]. Persistence of baculoviruses in the environment is mainly thought to be due to the OBs that protect virions from UV light and allow horizontal transmission. But there seems to exist another way for baculoviruses to persist in the environment at low host densities. Baculoviruses can cause sublethal infections, and so be vertically transmitted from adult to offspring [77-79] or may as well become persistent or latent [80]. A laboratory colony of *Mamestra brassicae* was found to harbour an occult infection by the baculovirus MbMNPV with expression of viral genes at a low level [81]. Later, Burden *et al*. demonstrated the persistence of this virus in naturally occurring field populations of *M. brassicae*. RT-PCR analysis showed the presence of *polyhedrin* transcripts in asymptomatic larvae, indicating a covert infection [80-81]. Similar results were obtained using *ie1* as a target [81]. Moreover, these studies revealed that covert infections could be induced to produce overt infections when infecting these larvae with another baculovirus. This means that the persistently infecting virus retains its ability to produce a lethal disease in the larva. There is accumulating evidence of persistent baculoviral infections. Kemp et al [83] detected baculoviral presence (CfMNPV, CfDEFMNPV and a GV) in laboratory and field populations of *Choristoneura fumiferana*. Also, there were baculoviruses (SeMNPV and MbNPV) identified in *Spodoptera exigua* populations that could be reactivated to full lethal forms [84]. A study in

RNA mediated defense of its host for its effective proliferation [63].

70 Current Issues in Molecular Virology - Viral Genetics and Biotechnological Applications

**9. Persistent infections**

persistent infection [74].

IMC-Hz1, derived from the corn earworm *Heliothis zea*.

Among nuclear DNA viruses, baculoviruses have developed a unique strategy to synthesize late mRNAs which consists in having their own DNA-directed RNA polymerase. This enzyme recognizes viral late promoters that are different to promoters responsive to the cellular RNA polymerase. By this means, the infected cell produces high levels of viral proteins at times of the infectious cycle at which the cellular protein synthesis is mostly shutdown. A late viral progeny with a distinct phenotype is embedded in proteinaceous occlusion bodies (OBs) that assemble after overexpression of the major OB protein. In order to exploit their high protein synthesis capacity, baculoviruses have been developed as vectors for expression of heterolo‐ gous proteins in insect cells. This system is continuously evolving to new biotechnological applications. However, there is still a lack of knowledge about the molecular mechanisms governing the complex baculovirus infectious cycle. A better understanding of these mecha‐ nisms would also benefit the development of baculovirus as biopesticides. To this regard, the array of viral factors involved in regulation of gene expression is an important component of the specific virus-host interactions that determinate the susceptibility to the virus of different cell types within a host and of different hosts within a range of insect species.
