1. Dear [Minister's Name],

I hope this letter finds you well. I am writing to extend an invitation for you to be the keynote speaker at our upcoming seminar, [Seminar Name], which will be held on [Date] at [Location].

Our seminar aims to discuss [topic of seminar] and we believe that your expertise and experience in [relevant field] would greatly benefit our attendees. We would be honored to have you share your insights and knowledge with our audience.

The seminar will be attended by [number of attendees] professionals from [relevant industry] and we believe that your presence would greatly enhance the value of the event for our attendees.

Please let us know if you are available on the mentioned date and time, and if you have any questions or concerns. We would be happy to provide more information about the seminar and our organization.

Thank you for considering our invitation. We look forward to hearing from you soon.

Sincerely,

[Your Name]

2. CALL TO ACTION in a closing part of a business letter is a sentence or phrase that encourages the recipient to take a specific action. It is a way to end the letter by requesting a response or a follow-up action from the recipient. For example, in the letter above, the call to action is "Please let us know if you are available on the mentioned date and time, and if you have any questions or concerns." The recipient is being asked to take action by responding to the letter and letting the sender know if they are available to speak at the seminar.