I've lost the automatic upgrade function

[Dave M G]

Ubuntu Users,

I'm running Dapper, and for weeks after I installed it, almost daily a 
little icon would appear in the top right corner saying that there were 
upgrades available.

Then I used Synaptic to remove some Nvidia related driver packages, and 
accidentally blew away some Gnome related packages.

Since then I have not been able to get the update utility back. I can 
get upgrades by going to Synaptic and marking upgrades to install. But I 
would rather the check was handled automatically.

I looked for update related packages, and found apt-watch, but I don't 
think that's it because the icon is different.

How do I get the automatic update feature back?

Thanks for your help.

--
Dave M G