You could also put "Horst Schuler, PhD", and include the German abbr. in parentheses

see also http://www.proz.com/kudoz/258266 and others

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Note added at 19 hrs (2006-05-21 10:20:10 GMT)
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Ian's comment below is, of course, correct. As this appears to me to be the signature line at the end of a letter, and would presumably be sent to someone in the English-speaking world, I would recommend leaving out the German title altogether, as it will mean nothing to anyone and it isn't common to use such titles in the signature line anyway (apart from the simple "Dr.", which would be followed by MD/DO/etc. only in the case of physicians). If this is *not* a signature line, you could consider keeping the German title and explaining that it is essentially a doctorate degree in the sciences. See also the extensive information provided here http://en.wikipedia.org/wiki/Doctorate and
http://en.wikipedia.org/wiki/Doctorate#Doctorates_in_Germany