Conditional Web Page required field validator

I have a web I inherited

There are two items which I need to link together

First...I can easily add the required field validator for either control but have it for the dropdown list only right now

What I need is that IF dropdown list selected value = "Yes"
THEN
The txtTravelCo_ is required to be filled in


<asp:DropDownList ID="ddlTravelAssign_" runat="server" CssClass="requiredDDL">
  <asp:ListItem Text="-- Select One --" Value=""></asp:ListItem>
  <asp:ListItem Text="Yes" Value="Yes"></asp:ListItem>
  <asp:ListItem Text="No" Value="No"></asp:ListItem>
</asp:DropDownList>


<asp:TextBox ID="txtTravelCo_" runat="server" ></asp:TextBox>