R - how to concatenate a vector of characters into one string of words

Question

My question is fairly straightforward if I provide an example, along with my desired output:

# input
these_letters = c("I", "f", " ", "a", " ", "p", "r", "o", "g", "r", "a", "m")

# desired_output
sentence = "If a program"

so pretty much, I want to concatenate all letters into words, separating words based on the blank character " ".

Thanks in advance for help