The present invention relates to the identification of bovine adenovirus sequence(s) essential for encapsidation and E1 transcriptional control regions. The present invention provides adenovirus expression systems, host cells and compositions comprising adenovirus vectors which comprise one or more BAV sequence(s) essential for encapsidation, as well as helper virus which express BAV sequences essential for encapsidation. The present invention also provides helper vectors comprising a BAV sequence essential for encapsidation which is used in a helper virus for propagating recombinant adenovirus. The present invention also provides adenovirus expression systems, host cells and compositions comprising adenovirus vectors which comprise modifications in BAV E1 transcriptional control regions. The present invention also provides methods for making adenovirus vectors comprising BAV sequence(s) essential for encapsidation as well as modifications in BAV E1 transcriptional control regions.