> x <- c("cult", "brother sister relationship", "word title")
> gsub(" ([^ ])", "\\U\\1", x, perl=TRUE)
[1] "cult" "brotherSisterRelationship"
[3] "wordTitle"
Quoting from pattern matching and replacement:
For perl = TRUE only, it can also contain "\U" or "\L" to convert the rest of the replacement to upper or lower case and "\E" to end case conversion.