Well, if the journal enforces some style and they do not support natbib
's features, it is their decision. I suppose the reason is to enforce the \cite
commands to always show the bracket number.
Still, asking the journal editor for a solution cannot harm, but I suppose he'll tell you: "You have the style, nothing else's allowed."
As well, be quite sure that the editors will be angry if you start messing their style without them seeing any good reason for doing so.
(I'm a graphic editor of one journal, too, so I bring some experience here.)
I'm not aware of a BibTeX style file for PNAS, but the Bibulous project does provide an easy way of customizing styles. For the style suggestions linked to by the OP, it took me only a few minutes to put together a complete style template to follow PNAS' requirements. Using the following main.bib database file
@ARTICLE{Neuhaus,
author = {Jean-Marc Neuhaus and Liliane Sitcher and Meins, Jr, Frederick and Thomas Boller},
year = {1991},
title = {A short C-terminal sequence is necessary and sufficient for the targeting of chitinases to the plant vacuole},
journal = {Proc Natl Acad Sci USA},
volume = {88},
number = {22},
pages = {10362-10366}
}
@INCOLLECTION{Hill,
author = {Adrian V. S. Hill},
year = {1991},
title = {HLA associations with malaria in Africa: some implications for MHC evolution},
booktitle = {Molecular Evolution of the Major Histocompatibility Complex},
editor = {Jan Klein and Dagmar Klein},
publisher = {Springer},
address = {Heidelberg},
pages = {403-420}
}
and the style template file main.bst (the lines below show the complete file)
TEMPLATES:
article = <au> (<year>) <title>. \textit{<journal>} <volume>(<number>): [<startpage>--<endpage>|<startpage>|<eid>|].[ <note>]
incollection = <au> (<year>) <title>. \textit{<booktitle>}[, vol.~<volume>, ][, <edition_ordinal>~ed.][, <null.if_singular(editorlist, edmsg1, edmsg2)>~<ed>][, <series>][, Chap.~<chapter>] (<publisher>, <address>)[, pp~<startpage>--<endpage>|p~<startpage>|<eid>|].[ <note>]
SPECIAL-TEMPLATES:
authorlist = <author.to_namelist()>
editorlist = <editor.to_namelist()>
authorname.n = [<authorlist.n.prefix> ]<authorlist.n.last>[ <authorlist.n.first.initial()>][<authorlist.n.middle.initial().compress()>][, <authorlist.n.suffix>]
au = <authorname.0>, ..., <authorname.9>
editorname.n = [<editorlist.n.prefix> ]<editorlist.n.last>[ <editorlist.n.first.initial()>][<editorlist.n.middle.initial().compress()>][, <editorlist.n.suffix>]
ed = <editorname.0>, ..., <editorname.9>
null = {}
OPTIONS:
edmsg1 = ed
edmsg2 = eds
compiling the main.tex file
\documentclass{article}
\usepackage[paper=letterpaper, text={6.5in,9in},centering]{geometry}
\makeatletter %
\renewcommand{\@biblabel}[1]{#1.}
\makeatother
\begin{document}
\nocite{Neuhaus,Hill}
\bibliography{temp}
\bibliographystyle{temp}
\end{document}
produces the following formatted result:

This provides templates for only journal articles and articles/chapters in books, but the PNAS website provides guidelines for only these two. Templates for other entry types are easily derived from the two shown here. (For example, a book
entry type template can be defined by adding another line
book = <au> (<year>) <title>. ...
in the lines below TEMPLATE:
in the style template file.)
Best Answer
There should be a call
in the document preamble.
Here's a test document, using the files in the distribution.